I think a lot of people attach the heat pad to the side of the tub to avoid direct contact with the hot spot. Since the egg crates will be vertical, this gives them a kind of 'retes stack' to select how warm they want to be without actually having to sit on the heat. The little babies will be down on the bottom eating frass and they probably wouldn't do well at 100 deg.

Your other option is to find a warmer spot to put the tub - I have mine behind the fridge because the compressor puts out a good amount of heat. They like the dark, too.