This is a box we make called the D8. It fits on the Rear shelf behind the back seat in all 996 type Porsche® Coupes and Targas. It also fits in the '05 Coupes. made with 4 layers of 7oz bi-directional cloth in epoxy with no flat sides, and extra thickness in the baffle area. Stiffened on the front side and with cross bracing on the baffle. It fits most stamped basket 8's with a magnet diameter of 5-1/2, like JL and Kicker.
This is the shelf on a GT-2®. No back seat but the sheet metal and carpet are the same as all the vehicles that have a Bose® sub box. When the box is in a car with a back seat you can't see it from the driver's seat. it is flush with the lip on the back of the seat. You can just barely see it in the mirror.
This is what it looks like in the car (the GT-3®). We sell it bare with only the grill so we can sell it cheap. If you want we can supply it with original Porsche® carpet and cover the grill. Internal volume is as big as we can make it, about .75.
We make these with fiberglass cloth in epoxy. We are trying to eliminate the polyester, styrene and ketones from our work flow. With a little prep it could be painted body color or covered with leather, vinyl or whatever. We send it "bare" that is, no screws, wire, carpet. There is a half inch plywood ring backing the mounting baffle to make it easier to attach the woofer. You can set t-nuts in that to make the mounting even more robust.