I'm working on an enclosure for the spare tire well. The enclosure will sit inside the spare, like the original Bose sub. The trunk board will need to be cut right above the speaker for best sound quality. The enclosure will fit a Pioneer 12" shallow mount sub. It's not done and I don't know how it will sound compare to the sidemounted ones. Here are some pictures. It should be done by this weekend.
http://community.webshots.com/album/370039683uGJapS
http://community.webshots.com/album/370039683uGJapS