This will allow you to switch out the pedal on the speed controller of the G9928 flex shaft grinder for a speed dial. This is another thing that came out of me working toward my own CNC machine design, since I'd like fine grain control of the spindle speed but don't want to build a separate controller and obviously don't want to operate the pedal by hand (foot?).
The G9928 grinder (and similar grinders that appear to be from the same manufacturer) is reasonably priced and is more powerful, more versatile, lighter (at least the tool head part), and easier to mount compared to most other rotary tools. The only downside other than having to convert the speed controller to dial operation is the fact that the tool head has a little bit if play in it. Not a lot, but more than I'd like to see in my final spindle setup, so I'm working on addressing that as well.
Cut everything out of 1/8" material (I used 1/8" ABS sheet). The 1/2" long #2 screws hold the handle pieces together and the captive nut components to the top of the case. You'll need to remove the pedal top and drill three 3/32" holes in the base of the pedal to attach the adapter with the 1/2" long #4 screws.