I lost the on/off/volume knob on my Peltor/3M Tactical 100 hearing protection. I made this replacement knob which fits very well. It's a little snug but goes on with finger pressure. The interior hole is 6.06mm. I also made it a bit longer than the original making it easier to manipulate.

I've included a smooth version which was closer to the original.

This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/bZSRN8SWpn8