I cut out the inner steel quarterpanel on my 4-door, and cut the fiberglass trim around the window. Fabbed up an enclosure to follow the contours of the space out of 3/4" MDF, then fiberglassed everything in place. Lastly, carpeted the enclosure, and made a plywood grille to go over the sub, preventing anything from sliding around and damaging the sub. About 2.4-2.5 cu ft internally, with the port displacement and sub displacement (Celestion AD15H), I've got just about 2.0 cu ft.
And I have one on each side..... still working great after almost 3 years.
Still have 49" from wheelwell to wheelwell, enough to slide a full 4x8 sheet of plywood (or anything else) in, and have it hang out over the back of the tailgate, or stick out the hatch. As an aside, if I've got something heavy, I'll support it by placing a 2x6 across the top of the two enclosures, which are even with the top of the tailgate - this way, I don't stress the tailgate hinges.
---------- Post added at 05:28 PM ---------- Previous post was at 05:23 PM ----------
Or, you could just build a center console, have the subs fire downward, mount the amp(s) under the seats.... I fabbed brackets to move my seats about 1 1/2" towards the doors, gave me almost 14" between the seats to build the enclosure (for two Powerbass 10's, and 3 amps mounted underneath).