I know many ppl suggest damp towels on screen tops to add humidity, but towels also let air thru, so they do far less than anything that allows no air flow...things like plastic,
foil, wood...all are better than a towel, except they hold no moisture on their own, you have to use the right substrates that retain humidity, &/or a humid hide, &/or a large
water bowl (& a water bowl over or near the heat will promote more evaporation & humidity will rise).