The heat tape, such as Flexwatt goes on the outside of the tubs, not inside. Most racks have the heat tape installed on or into the rack, with the tubs sliding over. Heat tape size depends on the size of the rack and tub. The heat tape is used to create a hot spot, not meant to raise the ambient air temperature.
Herpstat, is a popular thermostat and one I use. There are a lot of thermostat styles out there, makes and models. I would suggest purchasing one that best fits your needs.
If all is installed and used correctly, heat tape is reliable and safe.
Hope that helps
Thanks! Yeah, I know it goes outside-- but does it need 'breathing room' like heat pads do? I would feel better if it were on the back (of the bin) of an open backed rack, not touching any wood. But would that provide enough of a hot spot for the snakes?