I built a custom box for my rgular cab for one 10" sub. It's easy if you measure real carefully. There's room back there, just barely. The only thing is, you run out of room for an amp. You have to shop carefully for amps to find one with the correct dimensions to fit under the seat, cuz after the box is built, it gets harder to fit an amp back there too. IF you don't wanna lose any leg room. That's how I designed my sub box. To not lose one click of room.
Funny thing is, I ditched it for a Thunderform.