Fingertip control for electronic knobs

I'm currently driving a new digital transmission electron microscope. The knobs which control the X and Y motion of the stage are so fluid and slick that they're hard to use, especially since I have to cover a large area (at least 1mm sq!) looking for virus particles.

Thus, this thing. It's sized to press-fit my stage control knobs, and has a fingertip indentation underneath (which becomes the top). Prints okay on a Makerbot in ABS even with the overhang. So now I can rest my fingertip on the