I'm a bit late to the party but I think you may find this thread interesting as it contains complete instructions (with pictures) on how to make a D.I.Y. carbon filter.

http://boards.cannabis.com/grow-faqs...on-filter.html
PhatJay Reviewed by PhatJay on . Home made carbon filter: yay or nay? I'm looking for a cheap and easy carbon filter solution, and I think I may have built something that will work.Let me know what you think. As I've never used let alone built a carbon filter, I based this design off of what Ive seen other people build, so I may be way off base or (hopefully) somewhere in the ballpark. I took a basic duct coupling with a finned circular base (sorry, I don't know the technical or actual terms for these things, but here is a pic): Rating: 5