How about no "box" at all? If you have an adjacent attic, basement, crawl space, you can look into the possibility of an Infinite Baffle subwoofer. In the HT itself, the drivers can be mounted wherever, ceiling (if attic), etc, you could make a stealth grille for the driver(s), etc. I can't tell you if you were a good candidate for this, I've no experience, but am letting you know of the possibility just in case. It's what I would do if I wanted an "in wall" system, assuming my setup was a good candidate (my guess is that I would be, but that's not my chosen route).
