Honestly, I'd recommend a thermostat over a rheostat because they are going to be more accurate as the room temps change. For my corns, I can get away with a Hydrofarm thermostat (http://www.amazon.com/Hydrofarm-MTPR...2093006&sr=8-1) though it does allow for about 4 degrees variance before kicking in. With that on, I've never had any problems with a tub melting and I've used them for some time. For balls, I've heard it's good to get a more reliable/accurate thermostat such as a Herpstat, Helix, Ranco, or Vivarium Electronics. Most of those are proportional instead of on/off like the Hydrofarm variety which actually increases the life of your heat mat/heat tape. They also will keep the enclosure within a degree or so of where you want it.