Awe bud, do you know the rating of your fan? I tried something similar, but my fan isn't strong enough so it was getting warmer than I'd like. I'm using a 120mm, 87 cfm fan. Anyways, this is what I did. It might work for you.
Check my thread, http://www.420sa.co.za/index.php/topic,969.msg8708.html#msg8708 you'll see the box I made that houses the fan. At the other end of the ducting have a cardboard box that it dumps the humid smelly air into. In the lid of the box I have a small dish that I cut the bottom out of, replaced the base with pantyhose and put a layer of carbon in there. A stronger fan would obviously be better, but this works for me.
The box houses all my wiring, blocks the light leak from the ducting, muffles the humming of my fans a shit load and makes the cupboard "fit in more". In my garage the db reading with everything off is 54, with the fans on and the cab door open it's 76, with the cab door closed it's 57.