Actually, I've done something somewhat similar. What I did was take a large Glad ziplock bag, and simply (not really sure how to describe this) took each side and ripped it until all the ziplock bag looked like was a plastic sheet. I cut both the ziplock end off, so now I pretty much have a plastic sheet that was once a ziplock bag. I put it on my 20 gallon enclosure screen top, and was looking at how much I should cut off. I cut off some of the plastic sheet to make it cover only half (or a little less than half) of the screen top. I took a hot glue gun and glued the plastic sheet down. Now, my humidity does stay in pretty well. I did the same thing with my male BP's 10 gallon screen top. Also I glued the hygrometer meter to the middle (at the very top of the enclosure) of the enclosure because when I put tape on the back of it so it could stick on Candice kept knocking it off and what not so I was just like whatever I'll just glue it down with the hot glue gun.

Btw, is the humidity meter/gauge supposed to be in the middle of the cage at the very top of the wall of the enclosure? That's where I have it, and not too sure if that's correct now..