For heating and thermostabilizing a small volume or device, I can recommend the simplest way using an iron thermostat contact and a power heating resistor, e.g. a 25W or 50W 5-Ohm resistor with a 12 VDC power supply.
The iron thermostat can be adjusted to 200 C set as a tripping point and operates extremely reliably.
I have also designed thermostats with thermistor sensors, a LM311 comparator driving an IRF520 Mosfet switch in heater circuit. Sometimes I added an optical power-mosfet coupler to switch a fan, so the heater and fan operation alternated to keep the system temperature to +/- 1 deg.C.
But the iron thermostat is easily available, no electronics involved, and cheap.