one of two things will happen: 1. It will sound like complete crap. 2. You'll fry the sub.
If you don't want to go thru the trouble of picking out subs before the box and then figuring out some dimensions based on the speaker's needs and the measurements of your hatch, then just get a QLogic box and go with that. Get a small double sub box and use polystuffing to round out the volume reqs. It will fit behind your seats and your hatch will close.