A 10" can be made fit using a stealthbox type design. For a 12" you'd have to be more "creative"
as the fuse box and other parts will be in the way ....
It all depends on how "stealth" you want the box to be.
If you happen to speak to the folks at JL Audio you can also point them to Derek at Sound in Motion who's been building customer subwoofer boxes for various X5s (he just sent them one of the subwoofers out of my E90 330 to help them build a sub which fits into the space under the seats for the E90/E60 BMWs (stock location for subs)) ....
Good luck,
Jan