Well you're looking at at minimum 3 cubes per sub to make them sound good, so if you aren't worried about trunk space, you're looking at about 3ft x 3ft x 1 1/3ft box. You have two options though. Either rear firing & deaden your trunk, or front firing & seal the trunk off from the cabin completely.
Me personally, I'd say make a trapezoid-shaped box (in regard to the side) firing backward. That's just me though. Are you wanting a nice box or just functional?