There's a very old thread on hear if you search for it where the OP makes the filter from two pencil holders. Unfortunately I cant find it at the moment. But there are a lot of similar threads if you use the search bar.
I would suggest asking some of the folks in those threads if they were effective in the long run.
Personally I don't think pc fans can handle the draw through the carbon, but like I say lots of people have tried and would know better than me.