A carry-over post I made in honda-tech.

Fiberglass is a great way to preserve space in your vehicle while getting the bass you want and creating a professional install. I wanted to install 2 kicker 10" CompC woofers while still being able to transport cargo. (I install signs for a living) You can also apply this to other custom pieces, like gauge pods, speaker housings, or anything else that would need to follow the form of your interior.

>>> First: some warnings. Fiberglass resin and hardener is toxic, and the fiberglass itself is a major irritant. Wear long clothes, gloves, and a respirator/mask. If you get resin on your interior, it will not come back out. Mask your areas well past you work zone to preserve your interior. Fiberglass resin is noxious, the smell will stay in your vehicle for a long time (The smell really started to fade after 4 months). The more you can do outside of the car, the better.

>>>Step 1: Plan
Yes, you need to put thought into it. Fiberglass, especially for subs, is tricky. You need to be able to support the weight of the speaker as well as satisfy the airspace requirements of the sub and retain your cargo space. Find out what airspace you speakers need from the manufacturers. My CompCs are a little forgiving, sealed box volumes ranging from 1.0-2.4 cu ft. 1 cubic foot is approximately the size of an inflated basketball. I have not found a good way to measure volume before building a box that has complex curves, so take your best guess. Imagine a full sized basketball, sliced into pieces and laid into the area you have to work with.

>>>Step 2: Prepare
Gather your materials. I used 3/4" MDF board (available at lowes, home depot, etc) for the face of my sub. This provides me a flat, smooth face with plenty of strength to support my woofer. The back half of the box will of course be fiberglass. You'll need resin/hardener (about 1/2 gallon for my 2 boxes), fiberglass MAT (not woven cloth), cups for mixing, chip brushes (2" or smaller), masking tape (1-2" wide), Caulking, sound deadener (optional), and your safety equipment. OF course anything you'll need for an amp install and all of those wires as well, but this is more a fiberglass tutorial than a sub install tutorial.

>>>Step3: Make your mold
This is the first step of actually building your enclosure, and possibly one of the more important ones. If you mess up here, you could get resin all over your interior and rough or poorly fitting fiberglass form. Using your masking tape, make two criss-crossing layers of tape covering your intended work space, with generous overlap between strips of tape. Masking tape is moderately porous, you don't want any resin seeping through. Extend your tape coating a few inches past the edge of your envisioned enclosure to prevent resin dripping onto interior pieces. Following your resin instructions, mix small batches of resin and hardener and paint a moderate layer onto your masking tape. Once your tape form is painted in resin, cut small rectangles of mat (I used 2"x4") and place them on the resin, with small overlaps between mat pieces. Continue until your painted surfaces are covered in mat, then using your paintbursh again, dab/blotch (NOT brush) resin onto the fiber mat until your mat is the same carmelly color as your resin. Use a dabbing motion to prevent moving the mat pieces or creating poking out strings of fiberglass.

Let this dry, preferably in an open space. Once it has dried, carefully peel the mold out. If tape is left behind, that's ok. Remove stray pieces of tape that aren't well adhered to the fiberglass. There will be some pieces left on the back. Make another batch of resin and repeat the resin-fiberglass mat-resin method on the back. Be particularly careful with this fiberglass layer, as it will determine how well it conforms to your interior. Sand or grind off rough spots after it dries to ensure fitment is as true as possible to the original.

>>>Step 4: Beef it up
Add layers to your inside of the mold with the same resin-mat-resin pattern. I recommend another layer inside for small woofers, you may need more if it's a bigger sub.

>>>Step 5: Trim and cut
Lay your shell on a flat surface. Trim with heavy scissors until the shell lines up as close to flat as possible. Use a flashlight inside the shell and trace the shadow of any overhangs for a good line. Now lay the shell on a sheet of MDF or other wood and trace your shape. Cut the MDF to shape. Cut a hole in the MDF to your speaker size.

>>>Step 6: Attach the face
Using 4"x1.5" strips as a "tape", resin-matt-resin style, attach the face to the box. Sandwich the shell with your tape pieces for more strength. If you can wrap the tape-like pieces around to the face, that's even better, but no sweat if they won't stay adhered, sticking to the side will be enough since MDF is dense but porous to liquids.

If you have gaps between the face and your shell, bridge those gaps with this tape method, and add a third layer for stability.

>>>Step 7: Test fit
Now's the moment of truth, see if it fits! If not, you can trim and sand. Worst case scenario: If your shape is totally wrong, you may have to cut out the affected area, lay down masking tape, and start from the beginning again, incorporating your existing enclosure as you go.

>>>Step 8: Sound deadener
It's optional, but in my experience spray-on sound deadener on the back (non-visible) seals in the fiberglass smell and keeps interior panels from rattling quite as hard. Spray it on like thick spray paint.

>>>Step 9: Finishing
I used carpet, but if you used a grain wood like birch, you could stain or paint the face. Just pretty it up.

>>Step 10: Wiring
If you're going with a sealed box like me, you may want to finish your wiring before mounting the enclosures/subs. Take into account how your wires will get to the amp and how the amp will receive power and signal. Just basic install stuff. Sealed enclosures need to be airtight, so resin or caulking should be used to seal the hole that the wire goes through the box.

>>>Step 11: Mount the box
Screws from the inside will be the cleanest, and will make it difficult to steal. I just went straight into my plastic interior. If you're installing onto carpet, velcro could be a viable alternative.

Clean it up and you're done. fiberglass is a skill, it'll take a few tries to get it right, don't get discouraged! In this instance I added lights where the original cargo light was covered. Get creative with it.