I had the same problem with a fish tank with a screen lid. I covered the whole thing with thick aluminum foil leaving only enough uncovered for the overhead heater , moved the overhead heat near my hot side of the tank and placed the water bowl..which larger the better for humidity under the heat lamp. Also i use coco chip substrate which holds humidity really well. I make sure the coco chip is damp, not wet cause then you run the risk of getting scale rot if they are laying on wet substrate. In extreme cases, which i had to do because my tank was over 60 gallons, i got a reptile warm mist humidifier and that kept humidity around 60% most of the time. Cool mist works well too. Good luck...glass tanks suck but are doable lol