I actually built something a month or two ago that may help. I'll need to take pictures to show the whole of the build but I'm super tired today and not going to run down my camera etc today.
None the less I took my old school helmet. Ran a piece of small clear tubing around 1/4in. diameter through the top of the helmet and out the front right above the goggles in the center. I left around a half inch hanging down past the helmet edge. Then I ran that out a hole in the rear. Built a little circuit with a switch a double 18650 battery box, and a small squirrel cage fan from a computer around 60mm wide from memory. Then I adapted that to the tubing so it blows air through the tube and out the front when it's on. The batteries and fan ride on the back of the helmet out of the way, and you can run any goggle you want so long as there is a vent in the top to push the hose through when you put on the helmet.
I've run it for a couple games so far, and I need to harden the system it's pretty much proof of concept level now. But it works great, you can run any goggle with it and it's pretty sweet. My only complaint is it requires me wearing a helmet which is fine as long as I'm not sniping. I always run a helmet unless I've got the bolt gun, in those cases I typically run a boonie hat with ghillie mesh and veg etc. So I need to build another version that somehow works with that. But that's another day's project.
If guys are interested it's cheap and fairly simple to build, just solder some wires and buy some parts and you've got a functional helmet fan system for around $5 or so in parts plus of course batteries if you're going to run 18650's like I do.
I'd be happy to write up a thread later when I've got some time if there is interest.
Oh and I forgot one of my complaints on fan goggles was that the battery life sucked, like a couple hours. I suspect this setup will run several days though I haven't been able to test that yet. I have however run it for 8+ hours without any issues in game. So I'd say it's a win, even if I did have to change the batteries after a full day I'm ok with that;)
Luke