Depending on the manufacturer of the subs, you may be able to get dimensions on a box that is best suited for your particular subs. I just got a 12" sub from Resonant Engineering (500 Watts RMS, $185 shipped) and the SQL box I made per their specs just BARELY fits in the trunk of my '99 Taurus. Haven't got it hooked up yet. That comes Wednesday >)
My point is, try building your own if you can get specs from the manufacturer.