TL;DR

Mount and instructions how to mod your FAST helmet to fit antifog fan.

Required:

1x 5015 5V blower fan, like this https://www.amazon.co.uk/gp/product/B086HL2FGM
2x M3x25 screws + M3 nuts
4x (optional) washers
1x ~30x30mm adhesive velcro if you have velcro on your helmet OR double sided tape, 3M VHB highly recommended
2x rubber washers (optional)
Threadlock (optional)
1x sacrificial USB cable - length plan how you will plug it: powerbank on the helmet or run it down to a powerbank in a pouch etc.
~30cm of 10mm soft tubing for air duct - I've used extra I have left from my water cooling build
Some wide duct tape, soldering iron, scalpel or knife, screwdriver, pliers etc.

Assembly:

1. Put the velcro on the curved side of the "fan hub.stl"
2. Put screws with washers through the "fan frill.stl"
3. Replace the fan cable with sacrificial USB cable - make sure to observe polarity and test it before further assembly, DO NOT CONNECT TO PC, I'm not responsible if you screw up and fry your computer, use a powerbank. Put a blob of hot glue on the side where wires come out to create a strain relief
4. Put the hole thing through mounting holes of the fan
5. (Optional) put some rubber washers on screws on the fan side
6. Put on the fan hub part and secure nuts
7. Put the fan mount slightly off centre so that outlet is on the centreline of the helmet

Helmet modification

Depends on the helmet. Idea is that you run the tube towards the front of the helmet and split it in two so that air blows into goggle slits and displace the sweaty moist air on the inside of goggles.
I had a piece of styrofoam in front of my ASG Strike helmet, so I just carved the channel and secured the tube with some wide duct tape for time being - once I'm happy with flow and configuration - I'll use something like Sugru.
Don't cut through the whole styrofoam section - most likely you will render your insert unusable and ruin a helmet, my helmet's headband uses the insert to keep the helmet stable. Carve about halfway into it and make sure it is flat and smooth.

See pics for more details

Long version:

I play airsoft, I don't like mesh, cause it makes my eyes go funky and I prefer regular goggles.
I'm a big guy who likes to sweat a lot and NONE of the solutions I've tried worked: different antifog sprays/pastes/gels/wipes, dual pane aka thermal lenses etc - they either work to a certain level or at some point my sweat starts pouring down the lens and vision is heavily impaired.
The only solution that worked for me - active ventilation. A few commercial products exist already:

• ESS Turbofan - expensive, couldn't find in stock for 4 months
• Revision Desert Locust Fan goggles - less expensive, but also out of stock everywhere
• Exfog - not really goggles, does the job, but I don't think those thin tubes deliver enough air
• FMA fan goggles - I don't trust them enough, they're too cheap, safety ratings are shady, quite often QC is poor and goggles have quite a lot of defects

I've tried running in Valken MI7 mask with a fan strapped, worked flawlessly, but mask was too small for my mug and too bulky to use scopes or just plain look down sights.

Also, I bought a FAST helmet, so it was quite logical to build the ventilation into it