I used to have that problem and I have screened top viv's, I cut a piece of cardboard that covered 90% of the screen and taped it down all the way around the edge of the cardboard. I made sure the 10% that was open was on the cool end, so that the heat being created by the UTH was better trapped in the viv's. So far this has mainly solved my problem, you could give it a try. We have 2 in the basement and 2 upstairs. On the odd days that it did not a 50 watt red bulb placed on the area where the screen is open was enough to bump up the temp to the proper cool zone level but not over heat the viv. Much safer in my opinion than a 250 watt ceramic heater bulb.
Just an idea for you, if you want to try it.