I have an old tube sock (clean

) that I put some cedar chips in and sewed it shut. I just put it in the helmet when I'm at home and next time I put it on, the helmet smells cedary-fresh!
I saw something similar on a website they wanted about $20.00 for. The cost for mine was one old sock and $3.00 for cedar chips at pets-mart. If you know someone with a hamster, gerbil or other small rodent just snag a few handfulls next time you are at their house.