Nice work, J.Lane. If you want to cut overnight heating costs, you could replace the CHE with some 15W heat cord inside a hide box, or run it under that half log the CHE's sitting over. Even outside, this will keep temps 10C above ambient (on average) in cooler climates (Adelaide Hills, for example). No need for a thermostat with 15W, just some insulation across the base (styrofoam).
Thanks for the advice imported-varanus, it very rarely gets turned on. We let them shut down over winter and only put it on for a couple of weeks over that period. Not too cold here in South East Queensland.
Spencer's as adults are pretty hardy beasties also, I've kept them outdoors in western Vic without heating, just a metre of sand to dig in. having said that, I prefer to use supp heating these days for piece of mind.
Thanks BF. This was back in the early 90's (some of Greg Fyfe's first hatchies), so I only have these few photo's, unfortunately. It was a disused 5M round concrete water tank on the property, just pulled the corro lid off and filled it with 20 ton of sandpit sand. 2 M walls were difficult to access, and exit, especially with the male in full "food brain" mode.