I'm working on a homemade bator. I have a lamp kit wired to a water heater thermostat in a styrofoam cooler. The thermostat shuts the light off at 100 but it let's the temp drop to 90 or cooler before turning back on. What to do?
Try putting a brick or two in the very bottom, they act as a heat sink and will help steady the temp. closed jars of water will also work.
I have also found that mounting the thermostat in such a way as to allow air flow to the backside helps it react faster to air temp changes and narrowed my temp fluxuations from 11 degrees to 2 degrees. Hope this helps, Mark