Flex Watt is what we use as I build racks to keep the tubs in. With Flex Watt I can built a 12" X 11" heat mat for around $5. In our display type enclosure we use Zoo Med type UTH's since they are in the cage and thus are required to be water proof.
Get a Thermostat as the rheostat will require daily tweaking if not more! I have rheostats in a drawer that I can use as an emergency back up.