If you can, keep the horse in during the day and turn him out at night. His barn or shelter should be kept as dark as possible which keeps the flies and mosquitoes out. If you provide this your horse will likely stay in of his own free will anyway. Mine go in about 9am and stay inside until dark of their own free will. Small mesh hay nets are provided, otherwise 12 hrs is too long for them to go without eating. If you still decide to blanket, your best bet is one made of textilene, the same stuff as the stiff fly masks. It's breathable yet much tougher than the other fly sheets. Covering a horse in summer during turnout makes him itchy and he'll do what he can to relieve that itch, by rolling, rubbing on trees, fenceposts, etc. Life expectancy of said garment - maybe a few hours, a week if you're lucky.