So today I went out a got a small plastic tub, a brand new Acu-rite thermometer/hydrometer and a rheostat for the UTH. I placed the probe for the warm side on the bottom of the tub, stuck to the plastic. I figured whatever the temp was on the plastic surface, the temp on the surface of the substrate would be about 3-5 degrees cooler. I also made two smaller hides out of some cardboard.
Here are some images of the set-up...