The folks of Sound in Motion who build my subwoofer box and probably one of the first boxes which mount into the factory location without modification are actually selling this "smaller" (space for a 10" sub) box.
The cost for the box is appr 1000USD (and before people start whining you need to know labor for this type of box is appr +16hrs). The box itself is build from MDF (no fiber glass!) and will fit into the OEM location and held in place via solid mounts.
I have no commercial interest in this but just wanted to pass on some info for folks who might be interested in an alternative from a reputable shop delivering top notch quality work. Sound in Motion can be reached via
http://www.sound-in-motion.com/ or at 617.787.7744 (ask for Jim)
Below you'll find two pictures of the box itself
Hth,
Jan