A cheap alternative if you're missing or broke a knob.

These are very close to the real BOSS knobs. I included the small silver disc in the middle as well. It should just press into place. No glue needed. The ID on the bottom should allow you to press the knob onto the post with SOME force. I used PLA and the plastic did give enough to allow the knob to mold itself to the post. If you are having difficulty with this, try to remove some material by hand or increase the overall size of the part by 1 or 2%. All printers are a bit different.

As far as the white indicator, i just used a toothpick and carefully applied some paint into the channel. You can use whatever really... whiteout... paint marker... etc. or just leave it.

Print it right side up with support for the hollow section.