you need what i think is called "Free air subwoofers" i cant remember the actual name. most subwoofers you buy today are not and require a box. partially to make them sound their best, and partially to keep them from breaking i believe. ive always heard experts say that it is not good for a regular sub to run it outside of its box. it needs certain pressures, which is why box size is important to match to your sub. there are free air subwoofers out there though. my wifes nissan sentra came with 2 of them mounted int he rear deck.
other than that, unless you mount and seal it up real good, it looks like a recipe for rattles.
2002 GT 5sp. 16x,xxx miles
Steeda Exhaust, C&L plenum, BBK Throttle Body, Steeda Underdrive, Steeda short throw, unknown stage clutch. Rear main leak. Compressor seal leak. Steeda lower control arms. good tires.