I'm in the same situation as far as housing/growroom is concerned but luckily I haven't had to deal with smell yet- my plants are all still in veg. I don't know much about carbon filters, but someone did a post in the grow FAQ about how to build an odor eliminator using a bucket of odor neutralizing shit, and a small fan. They claim it's very effective, so you should probably check that out.

PS: if the closet is in a room with a window nearby, you might be able to run a ventilation hose across the ceiling or floor and out the window. it would look pretty bad if people could see the hose out your window, but maybe you could cover it up with a sarong, sari, or one of those bamboo/grass blinds.