I've got a cabinet I built that measures 36"x36"x72". I am using just 2 Lasko Box fans. Temps at the top of the canopy, which is 8 inches away, under a 400 watt HPS is at a steady 77 degrees.

I JUST NOW built a tray style carbon scrubber to go over the exhaust box fan. Air flow has come to a crawl but I'm willing to bet it still pushes about 300CFM.

Lasko rates this fan at slightly higher than 3200 CFM at it's highest setting.

The problem is that they velocity at which the air moves is not very fast.

I'm about to go check on my temps in my cabinet in about an hour or so. I'll let you know what the temps are with the carbon scrubber on.

btw.. box fans cost me 10 bucks a piece at Target.