I would definitely sacrifice a little trunk space. Buy one of the new thin subwoofers. Pretty much everyone makes them now (Rockford, JL, Pioneer, Kicker etc.). Unless you listen to a lot of rap or metal, I would go with a 10". You'll get a good amount out output and have a smaller box than a 12". I wouldn't waste my time with a bazooka tube, I've never really been impressed with those at all. The thin line woofers would probably be smaller anyway. JL is always more expensive, but the one here will give you an idea on the size.
JL Audio: TW5 Subwoofers
__________________
2006 Black Premium Convertible, Gray Leather, Pony Package, Shaker 500 with Clarion MAX385VD DVD Head Unit, JL Audio 10" W7 powered by a JL 500/1, TintedTail Lights