You might be able to get these to fit in a small box behind the seat but you won't be able to drill or mount anything extra behind there since the gas tank is right behind you.

http://www.crutchfield.com/S-6vWdDOR...0&I=130TSS2541

They are shallow subs (3" deep) and still provide the thump. It won't be as much as a typical cone subwoofer but it makes due for our limited space. I plan on making the trunk flush with 2 of them (almost like a second false floor or take the original floor out and replace it with the box that looks like the floor).

They're reasonably priced and should be a fun project.