You just need to find one of the boxes that is made for a truck, so it has the lean to go with the seat, and make sure the bottom is only like 8". I got really lucky, I had an old school bandpass box that was this exact size, replaced the sub in it with a new one and it takes up very little room, you probably cant do much better. Can still open the secret hatch in the back even without moving the sub.
Kicker and JL Audio both make boxes small enough to fit in that spot. Otherwise your looking at the tiny all in one Infinity Baselink or something, these are ok, but wont give as much bass. I didnt want to go over board, but was missing too much. Replaced front door speakers, put an amp and sub and its perfect for me.