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
Blue painters tape
T-shirt type material
1"x1" 1/2-3/4" MDF board
Wall Paper roller (6")
*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
Step 3: 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
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
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
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
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
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
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!