I'm considering building a 4x4x6 box now, just sketched it in sketchup and would require 10 2x2x8s and five sheets of whatever the cheapest 4x8 plywood is. home depot or lowes cuts that shit for free.
I already have a 400w light and one of those 100cfm duct fans on order so basically i need to size around that now. I want two compartments, one for veg one for flower so i figure a 4x4x4 flower and 2x4x4 veg will be reasonable. I'm going to veg with regular FL or CFL lights and the 400 watt MH for flower. I figure i can hook ducting from both the top and the bottom in a T and then put the fan for exhaust, and stick a small fan inside the flowering chamber to keep the air moving.
Ideally i'd like to flower 3-4 plants at a time so I have enough extra should some males surface, and hopefully 4x4x4 will be enough.