man don't buy a filter... make one. all you need is some 3" PVC, some screen, two "band fasteners" and charcoal briquet's.
1) Take one end of the PVC and place the screen around the end and secure with a band fastener.
2) break the briquet's into maybe 1/4 - 1/8 sized pieces (or smaller depending on best absorption)
3) fill the 3" PVC tube with the briquet's and wrap the remaining piece of screen around the open end and secure with band fastener. (trim for aesthetic value)
4) use this as a basic setup and connect to your external venting fan with some 3" dryer exhaust foil pipe. enjoy now you have a $7 charcoal filter that will work as well as any $90 filter.