Wooch is probably far more qualified to give a meaningful response in the area of sub placement. If you're stuck with one suitable position for a sub, I would still argue you can make do and enjoy a sub. You will probably get a boosted response having the sub so close to walls, but other than that, I can't offer you much help on this front without actually measuring the sub in the room. You might not really be able to predict much before you try the sub in your room.

As for the 15"X15"X21"...you can totally do a 12" in that...the 1 cubic ft cabinets used in the 10 Titanic kit are suited for 12" woofers as well and can be purchases separately with no pre-routered holes...take a look at the "sealed subwoofer" recommendation for an alternative approach to the Titanic kits...it uses the 14"X14"X14" cabinet in a sealed box...I don't know the parameters exactly, but I have little doubt you'd get usuable bass into the high 20's...using the Parametric eq would probably further enhance that...there's a lot of guys on the PE forums that have reported excellent results in this compact design, and apparently they're especially good for music applications. You'll need a plunge router and jig to cut out a 12" hole for the driver, and probably a drill if the other holes aren't pre-drilled. Just an option.