Ok lads, here's my grow cabinet. It's designed to take 4 x 42W CFL or a 150 HPS with 2 x 42W CFL. Currently I'm vegging seedlings so I put 4 x 13W CFL and move up to 4 x 26W CFL for early veg. The CFL lightbar makes this box very flexible.
The ventilation design uses a single 65CFM AC 80mm fan from RadioShack in the top section. The lights are separated from the grow chamber similar to a ventilated hood or a cooltube. The fan runs at rated flow because I don't block it up too much and have the intakes opened up to twice the exhaust duct size.
The grow chamber has a small scrubber that runs around 10 CFM. It doesn't need a high flowrate, just enough to change the air every few minutes otherwise a wind in the growbox just stresses the plants.
I picked this cabinet up a Crappy Tire for $50. It's 2' wide x 1' deep and 60" tall. There is 40" of growspace inside.
The top section contains the lights which are sealed off from the grow chamber with a sheet of glass. The scrubber is built into the duct grill. There is a passive intake in the floor of the box.
A sheet of glass separates the light section from the grow section.
This is the light section cooling fan. I've lined the fan chamber with this furnace filter material which works good to muffle the fan noise as well as providing a light trap.
The fan behind the furnace filter material.
This is the intake to the light chamber. It's 5" x 5" to match up with the 4" exhaust fan.
Here's a view of the intake 5"x5" and the 4" exhaust fan.
My scrubber is a cold air return filter for a furnace. I take the filter, spray it with rubber adhesive and shake on aquarium carbon. There is a 65CFM 4" AC computer fan mounted on the back of the cabinet.