HJC makes an airbox for their helmets. My friend has one, as he puts it, "Not only does my visor not fog up, but if I want to commit crimes it doubles to mask my identity."
I picked up the half assed version of his(mine was $10 so was his...I got ripped), and hate it, I'm currently looking for his, it plugs into the holes in the front.
EDIT: Here's the one he has(I'm ordering mine now):
http://www.amazon.com/HJC-CL-SP-CL-15-Breath-Black/dp/B000Y5ANZI
This is the POS I've got:
http://www.amazon.com/HJC-Universal-Breath-Box-Black/dp/B001A6PLHA/ref=pd_sbs_auto_1