Subwoofers are the easiest place to start. IF you screw the sub together and use glue you can separate with heat, you can rebuild the sub if it isn't quite right. The "basic guideline" is the size of the box is "square" based on the size of the driver... 15" driver needs an 18" square INNER DIMENSION box. Internal bracing is dependent on "how much air can this sub move"... IF you buy a sub with an enormous VC and magnet, and pump 1000 watts into it, you need to brace the box to keep the sub from "sucking it in"... Robert J just helped a guy build a box(the thread is here somewhere, if I find it before you do, I'll link the thread) based on Q and everything else. If you buy existing subs(like these CV D8) where finding out its "properties" is near impossible...you start with a square box. If you need to, you make numerous sides with different size holes for different lengths/thickness of PVC for the port. If you keep it Acoustic Suspension, you need a "slightly bigger" box, than you need with porting.