Goes without saying that you should have a fuse but not for control of the heat, rather to protect against faults.
Now is where things start to get more complicated and, as larger currents are involved, more expensive.
To begin with you need to measure the current that the circuit is drawing to get some idea of the size of any switching or voltage drop devices.
This hit and miss method you are using could go on for years which is why theory was invented. One alternative is make it variable.
Do you mean a variable resistor?