I work in the HVAC trade and I come across some pretty strong inline fans. 6", 7", 8". Heres the brand we use. http://www.rewci.com/fadl408cfm8d.html
Hell I could probably even rig that box that comes with it into something for the charcoal. With a homebrew scrubber though, the fan is going to be the most expensive part of it, and there has to be a quiet and powerful enough fan out there.

I drew up an idea I had here: http://img143.imageshack.us/img143/4672/scrubber5zz.gif

With my situation, i'm really not concerned withmy grow room itself smelling inside. My biggest concern is that people outside passing by, like the mailman or a neighbor doesnt smell it. But, I think this same thing would work if I wanted to use it IN the grow room itself to cleanse the air from the source. I am just trying to keep the electricity consumption down, so it would be one less fan motor running if I tie it into my exhaust.