https://www.instructables.com/id/Air-Cooled-Paintball-Mask/?ALLSTEPScheck this one out, the best one ive found.
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I made a fan for my paintball mask a number of years ago. Here's some observations.I mounted a small computer case fan to the inside of the mask, in front of my mouth, blowing out with a battery and switch located in the same area. The idea was that since your warm and rapid breathing is the biggest fog creating culprit this would draw air from under and behind the mask and carry away the moisture laden air before it could fog the lens. It worked decently well as I recall but probably could have been improved since I knew very little about electronics at the time and it was probably woefully underpowered.The mask I had jutted out enough around the mouth that I never had problems with any of the parts being in the way but a lower profile mask may rule this method out. The real problem was the noise. It was quite enough that opponents would never hear it but I could hear very little inside the mask.You probably know that most commercial mask fans mount above the lens. The problem you will have making this your self is making an enclosure. If you wanted you could model and get one printed but that might end up being more expensive than just buying one you know will work.@rickharris probably has the right idea, using something to coat the lens and prevent fogging that way. I think there is a solution involving vinegar and water. Also try the forums of a paintball website like specialopspaintball.com for solutions both mechanical and chemical.
A fan may be difficult to get working effectively. You have to consider the noise a fan makes, plus the fact that it could get hit by a paintball. Then there's the problem of where to mount it without getting caught in your hair, or obstructing your view, or damaging the mask and still creating some air. I think your best bet is to go to a store that sells motorcycle clothes and accessories. They have a cap (looks like a regular knit hat), that is made with a material to keep your head cool. They wear them under motorcycle helmets.Here's a link to show you what I mean, so check around where you live or order online from that website.
Having googled I guess you mean a goggle fan to keep your paintball goggles clear of condensation?Have you tried the antimist cloths and solutions you can get for car windscreens and glasses.Wiping raw potato on goggles (inside) then polishing clear helps as well.Divers spit on the mask to keep clear.You could try RainX it disperses water.A mechanical solution is going to involve vacuum forming the plastic shield and then finding a small enough fan to fit in it - Some computer processor fans are very small and run from 5 volts DC.making a good looking light framework will be the hardest part.
It would help if I know what it was - You obviously do. Explain please.