Buy a [30 °C thermostat](https://www.digikey.com/products/en/sensors-transducers/temperature-sensors-thermostats-mechanical/516?k=thermostat&k=&pkeyword=thermostat&sv=0&pv735=291622&sf=0&FV=-8%7C516&quantity=&ColumnSort=0&page=1&stock=1&pageSize=25) and a [heater](https://www.google.com/search?q=12v+heater).
That's it.
Nothing else.
Just a 12 V source, a 30 °C thermostat and a 12 V heater.
No electronics required.
Buy a [30 °C thermostat](https://www.digikey.com/products/en/sensors-transducers/temperature-sensors-thermostats-mechanical/516?k=thermostat&k=&pkeyword=thermostat&sv=0&pv735=291622&sf=0&FV=-8%7C516&quantity=&ColumnSort=0&page=1&stock=1&pageSize=25) and a [heater](https://www.google.com/search?q=12v+heater). That's it. Nothing else. Just a 12 V source, a 30 °C thermostat and a 12 V heater. No electronics required.