Foil treating a screen top is the best method I've found for humidity. I tried plastic wrap and glass on the screen, but foil works best and is cheap.

Thermometer probes should be placed inside under all the substrate, and if you have problems keeping them in one place, try a dab of hot glue over the probe. I use hot glue to keep probes and plants in place with my more active snakes. And it's easy to remove and replace.