Adding a dimming knob to a step up power supply
This is a portable floodlight project, and because the board is housed
  inside of the body, this trim pot is not adjustable during use.


Inspect the trim pot and if necessary measure it after removing it from the circuit. Get a conventional pot of the same value and maybe even a multiturn type so you can control things more smoothly. Solder it in place (using short wires) and it should work just fine.

If the trim pot does not give enough range then I'd strongly consider using a different controller.