A generic design UK locomotive power (throttle) handle to be used on a 6mm splined shaft potentiometer / rotary encoder such as found on the cheap chinesium macropads that can be bought on sites such as AliExpress for cheap.
Rather than pay silly prices for full size controllers, I designed this to replicate the type of power handle, found on many UK Diesel locomotives. It can be friction fit to replace the knob one of the potentiometers to give a bit of a realism feel, while keeping things in keeping with the size of these macro pads. 

Printed with PLA at a layer size of 0.2mm - 0.4mm, about 25% infill, with whatever support works best for your set up. Just remember to put a support blocker where the pot splines go. But whatever settings on your 3D printer work best for you, go with them. :)

2 files with one having a smaller knob!