You called and emailed atomic audio too?
And idk how much can fit on your back seat, it will be kind of ghetto, but I guess you could do that. I figured out that a box very close to your friends would work, but depends how big a box you can fit thru your door. I would shoot for a box with a volume around 10 cubes or 48 width x 22 height x 20 depth. Do subs up, port side (maybe). Aero ports will sound better and save you some space.
Almost all subs, besides some SQ subs or specialty subs, will work in a generic volume box. By saying this, I mean that MOST 12s will work in a 2.5 cube box @ 31 Hz, or MOST 15s will work in 4 cubes @ 29 Hz.