I have used Lexan aka Plexiglass on my snake's tank to keep in humidity. It works just fine laid over the mesh and the way my tank is designed I belive this to be the best solution if the humidity drops. Though I tested the humidity not just by a gauge but also by sticking my hand in. It feels like the air after a heavy rain and only drops slightly when my air conditioner kicks on. My tank does have a small waterfall running down the back which will keep the humidity higher.

I have also already plugged the holes you mentioned with silicone. I had already heard about thumbnails escaping that way. Though right now the plan is still for a single Fine Spot Leucomelas since I am as my brother joked, "A First Level Frog Handler"