Custom Fiberglass Subwoofer




About: I like to take things apart, sometimes they go back together sometimes they end up as something entirely different then where they started.

Want a subwoofer in your car but dont like the idea of a bulky square box in your trunk? Why not build a custom fiberglass enclosure thats molded to your trunks interior and looks factory installed? Most car trunks have "dead" space in the wheel wells thats perfect for fitting a subwoofer without taking up valuable trunk space. 

Working with fiberglass is not very difficult and can produce some great results when you need a complex or custom shaped enclosure. I will show you what you need to do to create your own subwoofer enclosure that looks like it came with your car.

Step 1: Materials

Here is the materials you will need. 
Fiberglass mat
Fiberglass Resin
Blue painters tape
T-shirt type material
1"x1" 1/2-3/4" MDF board
1/2" dowel
Mixing cups
Mixing sticks

Wall Paper roller (6")
Jig saw

Latex gloves
Old Clothing

*warning* Fiberglass and Resin are both very nasty and dangerous to work with. Wear old and long sleeve shirts and pants as well as a respirator. If you can "smell" the resin then your respirator isnt working. If you get fiberglass mat on your skin use cold water to wash it off or you will itch. Use Acetone to wash off resin coated brushes and rollers. Their is a good chance you will drip resin on whatever you are working on. Cover the work area with drop cloths. Don't wear any clothing you care about, it will smell or be itchy. I would recommend a cold shower after working with fiberglass. 

Step 2: Define the Area

Define the area you want to create the fiberglass enclosure in. Use 3M blue painters tape to mask off the carpeting etc around the area. This will protect it from the resin later.  This wil be the rear wall of the sub enclosure and make the enclosure fit perfectly into your vehicle. Make sure to mask beyond the area you plan your sub enclosure to extend. If you drip resin onto your paint or carpeting it is unlikely it will come out ever. 

Step 3: First Layer

The first layer of epoxy and fiberglass is one of the most important steps. The first layer captures the shape of your car and is the basis for the enclosure. Go slow and be careful when applying the first layer. 

Mix your epoxy according to the directions and apply it lightly to the taped area. First, brush on a thin layer of epoxy to give the fiberglass something to grab onto. Stick the fiberglass mat to the surface then saturate the fiberglass with epoxy. You can use the small wallpaper roller to smooth out any edges or bubbles. Leave it out to dry as per the instructions until you have a hard single-layer shell.

Step 4: Layer Shell

Remmove the fiberglass shell from the car when it has fully hardened. The rear of this shell will now be the back of your subwoofer enclosure. Currently it is far too thin to be useful, its nearly transparent! You will need to layer fiberglass and resin on the inside of the shell to build up the rear shells thickness so that it can be a rigid pannel that can withstand the sound pressure levels a subwoofer will place on it.

I added 7 layers to the inside of my subwoofer. The number of layers you will need will vary depending on your application. Make sure to let each layer cure fully between coats. Once you have about 1/2" layer for the rear of the subwoofer and it feels solid you should be ok.

If you are building a larger box or have a more square shape it is possible to combine wood, like MDF and fiberglass to make a box using wood for the larger flat sides.

Step 5: Check the Volume

Subwoofers are really two parts that interact to create a low frequency sound. You have the subwoofer driver or the "sub" and the enclosure. Each subwoofer driver has specific enclosure volume for sealed and ported enclosures. Since we are making a sealed enclosure we only care about the sealed enclosure volume of our specific driver. In this case it is about 1 cubic foot of space*. 

Because we have a complex shape for our sub enclosure, how do we determine the volume of the enclosure? A simple way to calculate volume for an irregularly shaped enclosure is to create a similar enclosure of a known volume (eg a rectangle) and fill it with packing peanuts, sand, or anything else that will fill the volume and is easy to manage. Fill the container of known volume then pour the contents into your fiberglass enclosure. Mark the leven that the material reaches in your enclosure with a sharpie. This is the volume your enclosure should be. 

*Dont forget to account for the volume of your sub and any support bracing etc that will be taking up "volume" inside the enclosure as this will count against the total volume.

Use a dremel or similar cutting tool to cut your enclosure at the predefined dimensions. I used a heavy duty cut off wheel and multiple passes to cut the edges off the enclosure. You can also sand the edges after if you want. In both cases make sure to wear all of the same protective gear as before to prevent fiberglass dust from getting in your lungs or on your skin/eyes.

Step 6: Create the Front Shape

By now you may be wondering how this ugly looking hunk of fiberglass will make its way into a nice looking sub enclosure that will complement your car. In this step you will be setting the front facing shape that will create the visible portion of the sub enclosure.

Their are a few ways to do this, you can certainly get creative here as to where you want the sub positioned and what direction it should face. Here is how I did mine.

You need to cut out a ring of wood for the sub to mount onto. This MUST be wood and not fiberglass as fiberglass will crack and not seal properly. If you have a jig saw or a similar tool you can easily mark and cut out the ring shape from 1/2" or 3/4" MDF. Since you will only need about a 1 foot piece it is likely you can buy a scrap piece from the store. 

The speaker mounting ring must be mounted to the sub enclosure in such a way that it can withstand the pressure of layering the fiberglass in the next steps but also be removable when they are complete. To do this I picked up a cheap dowel and cut it into 4 pieces. I used Hot Glue to create a sort of tripod support system that held the ring in the proper 3D location.

Next grab an old t-shirt or buy a yard of cotton fabric and stretch it over the face of the enclosure. If you need to, make adjustments to the speaker ring until you are happy with the shape of the subwoofer. 

Again, use hot glue to attach the cotton material to the outside edges of the enclosure and trim off the excess. Now is a good time to test fit the enclosure so that you have a good idea of what the finished product will look like. If you dont like how it looks then cut off the cotton, reposition the speaker ring and try again.

Step 7: Layer Front Surface

After you have added the stretched cotton material over the front of the enclosure you want to build up a thick layer of fiberglass to complete the enclosure. The front is very delicate at this point as it is only supported by a few dowels and hot glue. 

To give your self a sold area to work with you are going to coat the cotton with resin. Use a brush and make sure the cotton is fully saturated with resin. It is not necessary to use resin beyond the inside of the speaker ring as you will be cutting into this area later and it will just make things more difficult.

Once the cotton has fully dried you can start layering fiberglass like you did previously. Remember to go beyond the edges of the front and press the mat down with a roller to seal the edges. Again, its not necessary to go to far beyond the inside of the speaker ring.

I would recommend using the same number of layers as you did for the rear of the enclosure as for the front. Use a roller to smooth out and bubbles or ripples in the front. This is more important that the rear as it will show more. 

Step 8: Trim and Test

Now you should be completely done with working with the fiberglass. Your enclosure should like like a messy blob of resin with fiberglass edges. Now you will trim everything up and give the enclosure its final shape and surface. 

I like to start with the edges of the enclosure as they can be potentially dangerous when you try to move the box around. Again, use your dremmel, a jig saw, or sander to trim and sand the edges smooth so that the front and rear of the enclosure are flush. 

Use a drill, to drill a pilot hole in the center of your speaker ring then start cutting with a jig saw until you reach the MDF. Cut away the center of the fiberglass with the jig saw so that it is flush with the speaker mounting ring. Again, sand smooth any rough edges.

You should now be able to fit your hand inside the enclosure. Remove the dowels you glued in earlier and inspect the inside for debris. 

I drilled a hole in the rear of my enclosure to pass the speaker wires through. You can use a speaker terminal cup or whatever you want though. Make sure to seal the hole with silicone calk or something similar when your done. 

Now you are ready to test your enclosure! Load in your sub and wire everything up. It will most likely not be 100% sealed at this point but you will be able to get a good idea if the enclosure was a success or not. If you have any leaks around the edges you can add additional fiberglass to the inside or outside of the edges in small bunches. You can also add additional support braces from wood supported by fiberglass. 

Step 9: Finish

Now that your enclosure is tested to fit and function properly its time to make it look good. While you can certainly sand and paint the surface to a super smooth and shiny finish, thats an entire instructiable by itself and not the look we are going for. We want the sub to look like it was OEM installed and not an add on. 

Drive your car down to an auto store, craft store, or car shop and find some automotive fabric that matches your existing trunk fabric. Usually trunks are black or a shade of gray so it isnt very hard to get a close color. 

I used 3M super 77 spray glue to glue the fabric to the fiberglass surface. Do yourself a favor and buy the good stuff or you will be re spraying this later. 3M 77 is good to have on hand anyway. 

Spray both the back of the fabric and the fiberglass. Its ok to use too much glue in this case. Start laying the fabric in the center and use your (clean) roller to stretch and press the fabric over the curved surface. Sometimes a heat gun or hair dryer can help mold the fabric to curved surfaces easier. Make sure to go beyond the front edges so they dont peal up easily. When the glue is dry cut an X into the center of the sub mounting hole. Trim the carpeting so that it extends to the inside of the mounting ring. Use additional glue if needed. 

When you are done you can mount the subwoofer. Depending on how flat of a sufrace you have to mount your sub to you may need additional sealing tape. Use a foam tape or weather sealing tape along the edge of the speaker cutout to seal any gaps. Pre-drill your holes and use wood screws, NOT drywall screws.

Once your enclosure is together put it in your trunk and try it out. If this is a new sub make sure to break in your sub properly and gradually increase the volume over time. You will want to check the enclosure for leaks or rattles as well as make sure the sub is tightly screwed in place. Check back in a week or two as you break it in. 

Congratulations! now you have a custom subwoofer and you still have tons of usable trunk space! 



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41 Discussions


4 years ago

When do I cut or make the hole for the speaker terminal? I just started my first Fiberglass Enclosure last night. So im at the end of "STEP 3" I haven't taken it out of the car yet because i ran out of daylight yesterday and now its raining!!! :-(

1 reply

3 years ago on Step 5

It would make most sense to check for volume BEFORE making the fiberglass shape. If your corner area won't have enough volume to begin with, you've just wasted hours and hours of work and money on wasted supplies.

What should be step 1 is to put the peanuts in a bag that is your volume minus the subwoofer displacement and then place that bag into your corner area to see how deep or tall you need to target the enclosure before you get started. You may find that your target area is too small or that you have have to change to a woofer spec'ed to fit your available volume.

1 reply

The only problem with that is being able to keep the "bag" of shipping peanuts in the intended area with out shifting any of the volume any where else in the bag they are in. When this happens the volume displacement is off and you might not realize just how much until after the fiberglass shell has been constructed. If your a car audio enthusiast then it's quite easy to look at an area and guess if it will work or not. Always better to end up to big then to small. If it's to big, fill small bags off sand and secure them to the inside to displace to volume till it's correct.

Brad h

Question 1 year ago on Step 2

Why do you have to use epoxy and not polyester as epoxy can become expensive ?

1 answer
Brad h

1 year ago on Step 4

You could use u pika matt-mat to build thickness and makes it so rigid sandwich it in between laminates

That Dave Guy

3 years ago

I'm planning to build a similar piece for my 2013 Taurus but I would like to use a piece of MDF for the entire front rather than a "volcano" sort of ring. My goal is to have a flat surface from the edge of the sub to the edge of the glass. I think this would be a very clean/factory look. Your thoughts?

1 reply
pcmofoThat Dave Guy

Reply 3 years ago

It is very common to mix MDF and fiberglass as needed. The only reason to use fiberglass is to build enclosures in spaces that are not flat. If you want any part of the box to be flat MDF is cheaper and easier. You can still use it for the rear and sides etc.

The difficult part with your design would be attaching the fiberglass shell to the flat MDF. I would follow the instructions and make the shell, trim the edges etc then use the epoxy to glue on MDF to make an internal frame 1-2" thick. You can reinforce the MDF to fiberglass joint with additional layers of fiberglass. Then make a MDF front baffle and screw/bolt it into the MDF edge with nice screws or bolts and T-nuts. You can use glue/calk and make this permanent or use a foam/weather sealing gasket and make the front removable.

Another option would be to stretch the fabric and mount the trim ring in such a way that the box appeared flat though MDF is way cheeper than fiberglass to cover that area.


3 years ago

I have sold my Honda Civic that this subwoofer enclosure was built for. The enclosure (and sub) are for sale and should fit many civics in the 96-2000 range and possibly beyond. Message me if you are interested!


4 years ago on Introduction

I basicly did the same job a few times some years back, and have a few quick tips.

* Tape/mask off the seating area from the trunk, as airtight as possible and with as thick a plastic as you have. The resin will stink up your car for weeks, maybe even months - so try to keep resin work /inside/ the car, short. And ventilate, ventilate, ventialte. Polyester resin will stink through plastic though, so be prepared for this.

* When putting down the masking tape, go from bottom to top - like wall cladding. That way, if there is any small leaks between the tape layers, the resin cannot run down behind the tape (unless you force it there).

* If you have parts of the area where you need to shape things "upside-down", don't use particulary diluted resin, and be sparing with it. The tackyness is all that is holding it up, and it will easily come apart from the backing tape before it cures.

* Once you have a shape, extract it from the car to do more layers. If its very flexible, support it so that it holds its shape.
* Make VERY certain to not put layers on the outer side, and if fixing spots/holes, don't build up thickness. The outer shape is litterally a custom fit at this point. You also want to keep this in mind when/if it comes to mounting terminals.

* Curves are much stiffer than flats, so count them a blessing, even though they can be a PITA to work with. Brace larger areas with dowels, plastic rope, pieces of wood/mdf etc, rather than build a very thick enclosure. If an area doesn't flex by a hard thumb, it won't flex by a woofer.

* When using cloth, test your resin on a piece before "the real thing". Ideally, stretch it first so you know how it will act and sag once its soaked. While some materials like fleece, are theoretically ideal - it may repel liquids. You need to soak the cloth, not coat it.


4 years ago

You did an awesome job, man. I've made my fair share of mdf boxes. I just got myself a new truck two weeks ago, and your instructable has convinced me to ditch the mdf for my next box. I've already got my back seat pulled out to figure how much room I have underneath, and plan the layout. I'm sure I'll be revisiting your post when I start my box.

1 reply

Reply 4 years ago

Thats great. Please share the pictures when you finish!


4 years ago on Step 4

For strong fiberglass you need to be able to apply another layer while the first is still sticky. So letting each layer cure in between coats is not the way fiberglass is meant to be layered. You end up with a much thicker resin layer which is significantly weaker than the actual fiberglass.


Reply 4 years ago on Step 9

an "Air Hole" in a sub is known as a "port" and creates a secondary tuning that allows the subwoofers back pressure wave to be used to produce a lower frequency than the subwoofer might be able to produce alone. The two main subwoofer designs are sealed and ported enclosures. I chose to make a sealed subwoofer to enhance the recovery time of the sub making it punchier for music like Rock. I would need to adjust the box volume and select a specific port length and diameter to complement the specific sub specifications. Many applications are available to help with these designs.


6 years ago on Step 3

I figured out that for the first layer, mixing a small amount of acetone in with the resin makes it much runnier and allows it to seep down below the mat without creating air bubbles. Also, instead of "brushing" the resin on, dab it on, but be aggressive with it, and you won't have any air bubbles to deal with.