I made this to adjust and maintain the speed of the drill that powers my drill powered lathe ( https://www.printables.com/model/823680-drill-mill ). This will fit over any handle that is less than 46 mm wide and 72.6 mm deep.  It fits my Black and Decker 10 mm drill model DR202.It will likely fit many other models. 

Ergonomic: The applied science of equipment design, as for the workplace, intended to maximize productivity by reducing operator fatigue and discomfort. This piece of equipment allow the operator to use the drill for long periods without fatigue. Also allows precise adjustment of drill speed by compressing the trigger a fixed and constant amount.

Parts are printed with PLA using a 0.6 mm nozzle with a layer height of 25mm. 

I printed the frame part using a brim an oriented with the open end on the printer plate and used organic support only on the bottom side of the arch. The hole that the screw goes through is at the top.

A 4 mm nut should slide into the little slot at the top of the frame part. Use CA glue to fix the knob to a M4 x 35mm hex head machine screw. Make sure the screw is threaded the entire length.