With no good idea what screws this little pot wants, I decided to make a bracket to hold it in place and use available (larger) fasteners to attach it to the pedal box.
I grabbed a bit of sheet steel and wrapped it around an old number stamp that's close to the size of the switch. The fold is to space the switch out from the edge of the box and center it in the slot.
I'll solder pigtails to the pot and then dab a little silicone or Liquid Nails on it to hold it in the bracket.