A couple of subs come to mind: the three π and the four π subwoofer. The three π sub is a 12" woofer in a vented box and the four π sub is a 15" woofer in a vented box. You could also go with a four π-18, which is an 18" driver, but I don't think you'll need it.I'd probably use two or four of the three π subs and place them together, within 1/4λ of each other at the crossover point. Run 'em in push-pull pairs for even better performance, by way of lower distortion.
Your room is going to be difficult because it's small and rigid. You'll have problems with room modes no matter what kind of subwoofer you use. If possible, have dampers installed.