Roo pretty well covered it.
Lay your cable FIRST and just do like 10 runs each way.

Basically.........pick a class, build a "box" as big as possible whilst still allowing room for upto the maximum amount of amps allowed in chosen class.
If possible, build the box with a flat back wall.
Subs as close as possible to the passenger side of the car, try to get them stacked one on top of the other, as close as possible, also try to keep them off the floor as high as possible.
Port, make it as big as possible and as close as possible to the drivers side (Ie: dont want the port to start 10" in from side of car)start with a square port edge on the baffle.
Over engineer everything, don't use concrete, use steel and a mix of sand and resin instead.
Use the biggest subs you can fit on the baffle, think of it this way, 2 18s are equivlent to 3 15s

dont go no wall, u need batteries

Come play in Max 1-2