Cheapest Way to Filter Air in HomeBox XS???

so what cheap methods are they're on filtering air. I just dont wanna put out the money for the carbon filters.

you can make carbon filters yourself using carbon found a fish store and some pencil holders at wallmart and a sock. Check out the DIY threads and i'm sure you'll find how to make'em. There might even be a DIY vid on youtube. i'm gonna hook a few up to my 4x4x7 tent, once i'm ready.