What kind of lid should I have for my crabitat?

I just purchased 4 hermit crabs (about one week ago). I currently have a 10 gallon tank (soon to be 20 gallon by thanksgiving). There is no lid and I was wondering if i need a lid and what kind of lid would be approriate? (I read having a lid helps keep humidity in). I also have been struggling with keeping my heat and humidity up. I can't get humidty past 50% (and that's only when i put a wet t-shirt on top of my tank). Also my tank was at 80 degrees but it fell to 70 and now it's at 60 degrees. I have a UTH that I put on the back of the tank but it doesnt seem to be working (even though it is still very warm to the touch). I dont want to kill my crabs and I'd like to resolve this issue ASAP.