The best way to build a box is to use WinISD...its a free program that lets you plug in all of the parameters of the subs to fine tune the size and tuning frequency (if ported) of the box to get the sound curve you want. I'm assuming you mean that the box is being built to go against the back of the rear seats and facing towards the back of the car? Were you looking for a ported setup (a little louder and heavier punch) or a sealed setup (for tighter, faster bass)? Let me know what style box you're looking for and what year/model subs you're using and I'll run up a quick WinISD chart to give you an idea of what you can do.