ultimately you have to treat the lenses to keep them from fogging. I bought some stuff at a gun show maybe 25 years ago. I bought 2 bottles & still have it, I'm looking at one now. I doubt that it's still available.
However, I do know of a dirt cheap solution:
Baby shampoo.
Put one drop on each lens, rub it around & then buff the lenses clean with a soft cloth. Micro fiber is best, T shirt materiel works fine.