I've seen a number of extruder knob designs. I wanted to add my own personality to my printer and these have been great. They print very quickly and can be functional if desired. For me, they are decorations. R2D2 actually looks like a little droid printing a part, complete with sound effects. Noisy printers finally have a purpose! Check them out in action: https://youtu.be/eb2SBfCWmTI

I have included four designs and may add more. Right facing arrow illusion Star Circle ambiguous illusion Flower circle ambiguous illusion and my favorite - R2D2

Inspired and remixed from: https://www.thingiverse.com/thing:3169391 https://www.thingiverse.com/thing:4667420 https://www.thingiverse.com/thing:3176144 https://www.thingiverse.com/thing:3878237 https://www.thingiverse.com/thing:2911133 https://www.thingiverse.com/thing:2776404