Every hot water heater has the perfect thermostat for this purpose, actually 2 of them on the large units. Just keep an eye out for a hot water heater out at the curb, being thrown away.
If you are desperate and do not have time to wait, most hardware stores sell them. They are very accurate. I have 2 in my honey heater. One is set to 104 degrees and the other is set to 155 degrees for melting wax. Just run the power to a dual 110 house switch (the ones that use a standard outlet cover) and then run one wire from one switch to one thermostat and a wire from the other switch to the other thermostat. Both wires from the thermostats go to the light bulb. For 155 degrees you will need at least a 150 watt light bulb. A 75 watt light bulb will work if you only make a 104 degree heater.