fiberglass box

37Z

I'm sure others that have fab up a fiberglass box can respond (see chops post).

I would guess that 3 layers, depending on the thickest of the fiberglass cloth, would be adequate for the side & rear if the box is attached to the rear hatch indent. The front braced area (most use a MDF ring) where the sub woofer is mounted, may need additional layers. You could gelcoat the outside of the box so the fiberglass cloth does not show through. Other fiberglass posts add putty (bondo) and paint. You may want to cushion the backside of the sub box where it butts up against the plastic trim panel to avoid rattles/squeaks. Also, you should consider adding a mounting strap/velcro so it does not move for safety reasons.