We actually had our clutches in a cooler, with the bottom filled with about 5 inches or so of water. We used a submersible heater such as the ones used for fish tanks, and attached that to a herpstat.
We then had egg containers sitting at the top, with some holes drilled in the sides to allow the humidity in and they were sitting on the light diffuser - no perlite/vermiculite at all - humidity/temps always spot on. Never had any eggs dimple prematurely or mold, and had 100% hatch rate.