Molded Fiberglass Stealth Subwoofer Enclosure for the Honda Fit

Pick a rear quarter cavity to mold into. A common choice is the passenger-side corner behind the rear seat, with an amplifier rack mirrored on the driver side. Confirm the cavity clears fixed components; on the driver side of the Fit the fuel-fill hose and box occupy much of the space. Match the finished internal air volume to your subwoofer's specification: one builder's 12-inch Rockford Punch called for about 1.25 ft3 sealed, while another targeted roughly 0.75 to 1.15 ft3 (ideally near 1 ft3) for a Type R driver. Wipe the panel and carpet down with a damp cloth before starting.

Because you laminate directly against the interior, the masking is the single most critical step; cured resin permanently destroys carpet and trim. Lay down tape, then cover it with aluminum foil. Keeping the foil as the outermost layer makes it far easier to release from the hardened shell later. Vinyl protectant on nearby surfaces can stop tape from adhering, so clean those areas first. Coat the tape with paste wax or cooking spray so the shell pops off cleanly. Drape a blanket over everything else in the work zone, since drips travel and stick to anything.

Cut chopped-strand mat into small strips, roughly 3 by 8 inches, so the material drapes into curves without fighting you. Mix resin and hardener to the product's ratio, brush resin onto the masked surface, press the strips into place, and stipple more resin through until saturated. Spread the material well past the final footprint of the box; trimming excess later is easy, adding coverage after the fact is not. In cold conditions a work lamp helps the resin kick. Let it cure overnight. With a single layer the shell will be thin, translucent, and brittle, so handle it gently.

Once cured, lift the shell out of the car and peel away the masking. Set it back into the cavity to check fit; leftover tape residue can be sanded off at the end. Now build strength: add roughly two layers of woven mat and two of chopped mat, then cure overnight again. Aim for five or six layers overall, with a couple of extra layers everywhere as insurance. Thin shells are the usual failure point; heavy drivers such as JL Audio W7s have been known to crack under-built enclosures within months.

Trace the shell's contour onto MDF, then cut a bottom panel and side panel to suit. Dry-fit the pieces against the shell, adjust, then glue and fasten them together; scrap blocks work fine as internal bracing if you lack dowels. Some builders make the box bottom out of fiberglass instead of wood for a stiffer, better-sounding result. MDF supplies the rigidity a pure fiberglass shell lacks, and later encapsulating it in glass seals the wood against condensation and moisture damage.

Cut strips of stretch fleece and staple them from the MDF edges across to the fiberglass shell. The fleece acts as a conduit that carries resin between the wood and the existing glass. Mix a batch of resin, saturate the fleece, and lay chopped mat along the seams, realigning the frame as gravity pulls it. Once the seams go tacky, add further layers matching the shell's thickness, laminating both faces so the frame and shell fuse into one continuous piece.

Measure the driver's mounting radius and scribe a circle; an improvised fixed-pivot marker does the job. Cut the inner opening, drop the speaker in to verify fit, then trace the outer diameter for the ring. Suspend the ring at the depth and angle you want using dowels and dabs of hot glue. For a cleaner face, some builders stack two trim rings so the driver sits flush.

Drape inexpensive fleece over the ring and frame to form the outer contour. Staple it to the MDF and bond it to the glass with 3M spray adhesive. This layer dictates the finished silhouette, so pull it taut and be meticulous. Test-fit the assembly in the car; grind down any high spots at the rear so the box seats flush against the body.

Brush resin onto the fleece and work it in until it soaks completely through. Let it cure into a rigid shell, then reinforce with several more glass layers, alternating between the inside and outside faces to fill gaps and even out the wall thickness. Weave mat lays up with fewer trapped air bubbles than chopped mat and is easier to conform over compound curves.

Spray automotive underbody coating over the inside of the enclosure, and apply a second interior coat after the box is finished. This damps the fiberglass walls and noticeably improves the sound quality of a molded shell.

Thin two-part body filler with resin at roughly 6 parts filler to 1 part resin so it brushes on like a heavy paint; adjust for your brand and the ambient temperature, and add the correct amount of each hardener. Skim the outside, sand, test-fit, and repeat until the surface is fair. To cut the sanding workload, prefer a brushable resin-and-filler slurry over piling on extra chopped mat and thick bondo to build up the exterior.

Wrap the completed enclosure in carpet, suede, or a stealth cloth so it blends with the interior. For the electrical pass-through, fit an inexpensive terminal cup (well under five dollars) or simply drill a small hole and seal a wire through it with silicone; a smaller hole seals better. Mount the driver and secure the enclosure firmly in the cavity before final installation.