i have had about 30 different boxes in my splash(one of the perks of being a car audio installer hehehe)... everything from a single 10 to 4 12's to 8 10's up to 4 15's.. I wouldn't go bigger that 12's.. my personal favorite is a ported enclosure... and i always build the boxes myself.. If you are gonna build a box yourself DO NOT USE PLYWOOD PARTICLE BOARD OR FLAKE BOARD!!! Use a good quality MDF minimum of 3/4 thickness. If you want to go above and beyond there are a few companies that make a special acoustical grade plywood... its 13 layer cross ply birch.. its heavy and very expensive.. about $90 for a 4x4 sheet. This is the only type of plywood that will last and sound good.