Cage/Feeding aggression is a myth. They do not learn to associate your hand with feeding time. I understand your concern about the aspen, but if you keep your prey dry it really won't be a problem. These animals can digest hair, teeth and bones...a little substrate isn't going to hurt them.

For heating I would suggest that you put your UTHs on thermostats, if they aren't already. UTHs don't effect the ambient temp in the tanks, but they can provide a warm spot on the hot end of the tank. Most folks try to control the overall ambient temp in a tank with a combination of heat lamps and abient room temp.

Optimally you will want to measure the ambient temps on each end of the tank, and use a temp gun to measure surface temps under both the warm side and cool side hides.