Covering the screen lid with some kind of plastic material will work wonders for retaining humidity AND heat.

In the past, I used heavy-duty plastic wrap fixed with black duct tape. Spend some time with it, you can make it look fairly decent. Others have had plexi-glass cut to fit the lid. Since you use an undertank heater, you won't need to leave any room on top for a heat lamp. You may wish to leave a few slots on top for ventilation, however.

I've got some old pics of lids I made when I used glass tanks. PM and I'll send them to you to give you an idea. There is also a Glass tank How-To in the husbandry forum that covers it fairly well.