An under tank heater will heat the frass and cause them to bury themselves to warm up. I'd just keep it on a thermostat and put it up on a wire rack or set it up in a way that allows airflow underneath the heat mat to protect against overheating.
I currently use a heat projector over the cardboard housing section of my colony and it works really well. Stays dark while projecting infrared heat through their housing. I think more of them can get warmed up this way.