Picture of Custom Fiberglass Subwoofer
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.

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Step 1: Materials

Picture of 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

Picture of 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. 
JohnSnow8729 days 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.
pcmofo (author)  JohnSnow8728 days ago
Thats great. Please share the pictures when you finish!
greenteagod4 months ago

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.

rajthilakk5 months ago

Subwoofer boxes usually come with an Air hole, dont we need that in this case?

pcmofo (author)  rajthilakk5 months ago
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.
rajthilakk pcmofo5 months ago

Thank you for the reply. That helps

Brabramsp1 year ago
What kind of epoxy to use??
ninjatails1 year ago
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.
bremus1 year ago
Good job. Thanks for this.
bLiTzJoN2 years ago
Will be faving this. Since you went from bandpass to sealed, you'll want to contact the manufacturer or look at the documentation to determine the recommended enclosure size for the driver or add a port to tune. I would also throw some loose polyfill in the enclosure. Another trick I used to do when making standard MDF enclosures, is line the inside of the enclosure with underbody (or bed liner) spray to give it additional durability and sound deadening.
My thoughts exact.
shanghei2 years ago
How is this secured so it doesnt move around?
pcmofo (author)  shanghei2 years ago
The weight and the shape of the wedge keep the weight back into the rear. Also the carpet in the trunk provides enough friction to keep it from sliding. I was considering velcro etc but it was completely unnecessary.
hamsammy2 years ago
Looks great! I'd like to do this for my girlfriend's car which currently has a way too large ported box. About how much would you say you spent on materials alone?
pcmofo (author)  hamsammy2 years ago
I think about $150 for the materials.
newroger2 years ago
gostei muito otima ideia parabens vou fazer igual no meu carro!
rhobere2 years ago
To further increase the rigidity, use less fiberglass, and decrease the amount of time you're sitting and waiting to cure, you can incorporate nylon rope in between the layers of glass. Adding resin to the rope will cause it to harden just like the fiberglass does, but since its so much thicker, it holds its shape a lot better.

Once you have two layers of fiber glass on your shell run multiple, evenly spaced "ribs" of 1/4" thick rope along the contour of the box, holding them down with a few dots of hot glue just to make sure they stay in place while wet. Add resin, then without waiting for it to cure, add another layer of fiber glass and make sure to contour it around the rope. Let cure and continue with another layer or two depending on your situation.
thewhite2 years ago
Absolutely brill! The idea, the execution, the final product. Very well done indeed. I'm not the biggest fan of sub-woofers, and other car-related stuff, but how you made it is classic. Also in my opinion needs to be some sort of netting/grille to protect speaker diaphragm, otherwise you need always remember to not chuck stuff willy-nilly into the boot.
bsodergren2 years ago
Instead of "T-shirt type material", you should use Fleece. It is thicker, absorbs more resin and is can be cheaper.

1 yard of it is only 5$ at wal-mart.
pcmofo (author)  bsodergren2 years ago
Looking back at my notes I actually used fleece. I will correct the instructable.
Panty hose also works too, and is useful in small jobs such as door panels and "A" pillars where thicker material like fleece and t-shirts are too bulky. You can use MDF as a base, staple through it and then resin over it. After it dries you can sand off the rough edges.
yoyology2 years ago
A beautiful piece of work!

Would you do anything to secure the whole thing to the side of the trunk? I'd be afraid that it would walk around from vibrations of driving and/or the sub itself.
I have found out that regular velcro sticks to car carpet very well. I use it on the bottom of my sub, air compressor, and toolbox in my trunk. I no longer get a THUMP! whenever I hit the brakes hard.
pcmofo (author)  yoyology2 years ago
Very simple, I was worried about the same thing. Because of the "keyed" shape of the enclosure it only has 1 direction to move, laterally across the trunk.

Further more, being on the right side it would only move in that direction during right turns. The box however is a slight wedge shape and nearly impossible to tip over. The weight of the box combined with the friction of the carpeting dont allow it to move much if ever. In some trunks, the lid supports may drop down and act as a stop anyway.

Which reminds me, Make sure your trunk lid closes before you finish the front layout!
WOW! did that come stock ? ;)
great job :)
pcmofo (author)  curious youth2 years ago
Hah! Thanks.
WOW! did that come stock ? ;)
great job :)
This is freaking awesome! Looks good!
nice job!