I wasn't thinking about going into the wall. I mentioned that I was short and had to have the seat up to clarify that I think there would be enough room for a small box. I was thinking of a box similar to this that fit behind the seat. 2 seperate boxes if I decided to go with 2 subs. From what I've read, these Pioneer subs hit pretty hard.

http://www.youtube.com/watch?v=WikkpS1sVdE