Maybe an electric oven capillary tube type thermostat would do the job. I realize it's just buying a part, but if you check the price of a solid state relay, along with the rest of the circuitry involved, I bet the cost is close.
A suitable enclosure, a cord, and an outlet would be all you need after that.
Any appliance repair place will stock oven thermostats.
If you found the exact temperature you needed, you could get even simpler with a bimetalic snapdisc type thermostat. It would then just be the snap switch, and an outlet.