You can't fit a 12" under the back seat w/o it extending into the feet space. You could probably fit a 10" if you angled the back of the box to go over that hump on the floor in the back. I was lazy and didn't feel like taking the time to do that much measuring, so I built a box that fits 3 8" subs aimed upwards. When the seat is folded down it covers them up. If I made it like an inch narrower it would be perfect.
But still turned out well. I'll try and get you some pics in a little.