The thermostat is one of the most essential parts of the incubator. It also is usually one of the most expensive.
I have made adjustments to the thermostat I supply so it works at incubation temperatures and holds a tighter hysteresis. (termperature range)
The thermostat operates a relay which opens and closes the circuit to the bulb at specific temperature ranges.
This keeps temperature in the incubator within one degree either side of your desired temperature.