Look into "PEPI" brand thermal switches. In your search you will also find a number of other types as well. However, PEPI are the most compact. They are all reasonably priced. The PEPI's are typically smaller than the other brands.
These are as simple as it gets for thermostatic control. It is simply a mechanical switch with two wire leads, which switches on and off at a fixed temperature. So, you have to choose the model of the switch according to the control temperature you want. They come in a wide range of temperatures.
You can often use these switches with no external circuitry. It depends on how much load current you need to control. Do not exceed the PEPI switch rating. If you need to control a larger load, use the PEPI switch to drive a relay and use the relay contacts to drive the load. Choose the relay accordingly.
You do need to understand a little bit about "trip points" and "hysteresis" to use these types of thermostatic switches effectively and correctly. You can find lots of info on this topic on-line. However, simply put, the thermostatic switch does not turn on and off at exactly the same temperature. Typically, they turn ON at a slightly lower temperature than their "rated" temperature, and OFF at a slightly higher temperature. These two, slightly different temperatures are typically called the "Trip On" and "Trip Off" temperatures or "points". The difference between them (in degrees C or F) is called the "differential" or "dead-band". Also, known by the fancy word "hysteresis".
The dead band plays a big role in how accurately your temperature controller will operate in the real world. The wider the dead band the less accurate will be the temperature control - in the sense that the temperature will cycle up and down as the switch cycles between the slightly different trip points. You'll get the idea once you hook it up and monitor the temperature of your controlled object with a digital thermometer or similar.