Disclaimer: My carpet is nasty (half the reason it's out) and the subs are old... (Click pics for full size.)
Here are the boxes sitting on the carpet in my garage. These end up flush with the front of the seat (more or less)
Driver's side enclosure. You can see how tight the sub is squeezed in. There is about 2" more to go between the box and the tunnel, but I wanted the boxes to be sized exactly the same (The tunnel is off center).
This is the bottom side of the same enclosure. You can see where I found room. I only carpeted the front, top, and sides of these because they will be bloted in from under the truck, and the backside will not be visible. The big hole is for a binding post plate.
If you can live with a couple of inches of box hanging out from the seat, you could probably do a 12. These boxes have a net volume of about .8 cubes, so keep small volume 12s in mind.