Replace a trimmer potentiometer with a thumbwheel (rotary) potentiometer
If you are using POT to control the output voltage in the LM2596 standard circuit then any of them will work. The feedback pin current is in 100nA range, so, power dissipation will be very small.

You can check the datasheet and you will find the standard application circuit for this buck regulator.



You need to calculate and adjust the POT value may be in combination of resistor value for R1 and R2 such that for maximum and minimum value of pot, you will have a variation of around 8V to 12V or whatever you need at the output voltage.