Try FogTech from Airsoft World, Cat Crap (not sure where to get it from) or try your local opticians. I use Leader Fog Buster from the opticians on my shooting glasses and that keeps them clear at least until lunch, then re-apply before the afternoon session. As it's in a pump action small bottle it's easy to carry in your bag or a pocket. You can then use anything that has clear plastic lenses and should not fog up during use.