i have tyhe same problem. I live in new england (ct) and winters are cold. i have to mist all the time as well, and its hard because i work monday through friday. I have a heating pad, and that doesnt do it by itself, so im forced to use an infrared bulb. which drys out the tank too. i did the plastic wrap thing, helped slightly. my pac is never out of the ground always burrowing and i think its because of the humidity issue. so this is what ive done and have noticed a large difference. so much so my pac has stayed out from burrowing for two days straight. even went to her water dish and sat for 45 min.
at lows they sell plexiglass. i bought a piece that was 14 inch long and 12 inch wide. well i scored the edge with a razor blade a couple times till it was deep enough that i could snap the excess off. i have a ten gallon to the 14 inch length fit good perfect actually as to still leave space for air. i cut it width wise just enough so it would sit on the inside lip that fish tanks naturally have. then i just sit the caged lid on top. the plexi glass isnt thick enough to disturb the cage cover and it sits perfect almost as having the tank have four sides. now my heat stays in there. another option would to be get the measurements and go to a glass place and have them cut i piece of glass to sit right in on those notches.... when i get home i will take photos to show you as well. good luck