Rickenbacker guitars back in the 60's had these cool Rogan Oven (or Stove) style knobs, which were made famous by John Lennon. Here's a version that you can print. In fact, I used them on my 3D printed Rickenbacker 325 homage electric guitar:
You can also download the STL files for the pickguard, guitar body, and neck at the link above, as well as find a link to a YouTube video about the design and construction of that guitar, and see a four song performance featuring it.
Other guitar knobs I've designed:
I printed them in black PLA, then switched to gold for the last millimeter, so they are top in a contrasting color, similar to John lennon's.
I capped them with a layer of clear doming epoxy for a nice finished look, but this is optional.
Also, you may have to drill out the center hole if its too tight for your potentiometers.