I got a ready made flex watt tape mat from West coast roaches, because I didn't want to be bothered making one myself. It works great and came with a control setting. At first I put a piece of cardboard between the mat and the carpet just in case it melted the carpet, but it didn't. Then, I found a piece of polystyrene and used that instead. Much better insulation, and the temps have been higher since. About 80 to 83 degrees on average. The mat is at one end of the bin under the eggcrates, and the food is at the other end.
I don't have that large a population yet, but they seem very happy and are breeding! I also have too many adult males, so I need to find a stomach for some of them. They're much too big for my little girl!