cheap way to filter air

what is a good way to filter the air comeing out of the grow box without spending a fortune? homemade is good too.

A car after market intake filter, the clone looking ones. Active carbon, I just saw some at Wal-Mart I didnt check the price but im sure it is cheap. A inlet fan, and some dryer vent flex metal hose. And there you go, one cheap effective carbon filter.

may be this cpould help you... i worked for me!!

http://www.instructables.com/id/Activated-Carbon-Air-Filter/