Fiberglass False-Floor Subwoofer Enclosure for a Honda Accord Trunk

Polyester resin fumes are strong, so work outdoors or in a well-ventilated garage and wear an organic-vapor respirator the entire time. Pull on nitrile gloves rather than latex, which the resin degrades. Keep a jar of acetone at your station and park each brush in it between resin batches so the resin cannot set up on the bristles. Tear the chopped strand mat into working pieces rather than cutting it; frayed torn edges blend into the next layer far better than clean-cut ones. Start with pieces sized to the tire well for the first few layers, then switch to smaller patches, roughly 4 by 9 inches, for detail areas.

Empty the spare-tire well and surrounding trunk pan completely. Lay aluminum foil over every surface the fiberglass will touch and tape the seams down with painter's tape so no resin can seep onto the car's body. This foil-and-tape layer is a rough release surface: it works, but expect it to bond more stubbornly than a proper mold release, so keep the coverage continuous and press it tight into the contours you want the shell to copy.

Set a piece of torn mat into the well where you want it and brush resin through it until it is fully wetted out and dark, working the resin in with a roller to drive out air. Build up two to three layers across the floor of the well, overlapping generously and adding extra mat around the rim where the shell will carry load. The lay-up warms noticeably as it kicks off; that exotherm is normal and signals curing. This molded shell becomes the structural base that everything else is built onto.

Let the shell cure until it is fully rigid, then lift the mold out of the well and peel away the tape and foil from the back. Test the result by standing on it: a properly laminated shell is surprisingly stiff and will hold your weight without flexing. If it feels thin anywhere, this is the moment to add more mat before you commit to the frame.

To hit the target of about 1.5 cubic feet of sealed airspace, use a displacement trick instead of guesswork: fill a trash bag with a known volume of packing peanuts and shape the bag inside the shell until the enclosed lump matches the box size you want. Size the wooden frame slightly larger than that lump to leave room for the thickness of the fiberglass walls, the resin buildup, and the volume the woofer cone itself displaces once mounted.

From the 1 by 1.75 inch stock, build a frame that defines the walls of the box on top of the cured shell. Set the frame onto the shell and anchor it with a couple of drywall screws driven down into the wood to hold its position. This frame gives the upper box its shape and provides solid edges to laminate against and, later, to screw the MDF lid into.

To wall off a hidden compartment for the capacitor and distribution blocks, stretch an old t-shirt across the opening and staple it taut to the frame; the tighter you pull it, the cleaner the resulting surface. Brush resin over the fabric so it stiffens into a rigid partition, then glass it fully to seal that pocket off from the woofer's airspace. This same tacked-fabric method works anywhere you need to create a wall between two enclosed regions.

Bond the frame to the base shell with fiberglass and keep adding layers until the enclosure is closed on every side except the top opening where the sub will mount. Add one more layer over the outer surfaces that do not contact the tire well, and build three to four additional layers on the inside of the woofer chamber, since that internal skin does the real sealing and stiffening work. Inside the hidden compartment, laminate several scraps of half-inch wood so the distribution blocks and capacitor have something solid to screw into.

A sealed enclosure must be airtight, and hand-laid corners often leak. Fill the box with water and watch the seams for weeping. To seal any leaks, mix a batch of resin with its hardener and stir in body filler, without the filler's own hardener, until it reaches a thick pourable slurry the consistency of a milkshake. Pour it inside, slosh it into every corner and seam, and let it harden. This flows into voids that a brush cannot reach and reliably kills corner leaks.

Cut a lid from the 3/4 inch MDF. Lay a fat continuous bead of silicone along the glassed wood frame, then set the MDF down onto it to form the airtight seal. Pre-drill and countersink the lid's mounting holes before driving the screws so the panel pulls down flat without splitting and the heads sit below the surface. Because it was cold, allow a few days for the silicone to fully cure before handling the box.

Set the finished box back into the tire well. If it rocks or wiggles against the trunk pan, spray black undercoating onto the underside to add a rubbery, slightly compliant layer that fills the gap, damps vibration, and locks the box into a snug fit. Fill any screw-hole countersinks on visible surfaces with body filler and sand flush so they disappear under the carpet later.

Cut an MDF ring so the woofer drops in flush with the baffle surface, and countersink the ring's mounting screws. For a secure, repeatable mount, drill the woofer bolt holes and press t-nuts into the underside so the driver threads into metal rather than raw MDF. Fasten the sub with stainless steel allen-head bolts into those t-nuts.

Wrap the top in speaker-box carpet: staple one edge, mist both the carpet and the panel with 3M Super 99 adhesive, let it flash off until tacky, then stretch the carpet across and staple the opposite edge. Flat-black any MDF edges beforehand so exposed seams blend in. Once the adhesive sets, cut out the woofer opening, spray a little more adhesive in the recess, and slit small X's over each bolt hole so the fasteners pass through cleanly. Wire the sub and connect it to the amplifier to finish.