i was thinking about doing the same thing but with a 12 inch sub. The problem i found is the "ripple" in the back wall will not seal easily and make for a reduced quality in sound.

I was thinking of building a sealed box in the back and then porting it through to the cabin.

I was going to weld a brace in the back aswell just to make sure i dont loose any structural rigidity.

Furthermore you could always use an Alpine Bass engine if you only want the vibration the subs make. will fit nicely under the seats.