I made it for my Ruger Precision Rimfire since the original knob is too underwhelming for me.

The pink knob (that doesn't look like a testicle) was printed with "fuzzy skin".

Depending on your printer you might want to print it just a teeny bit larger. I got my knob mounted, but it's probably cross-threaded. On the other hand, I'm not planning on removing it ever, and so I've loctited it that way.