One of my personal favorite designs is my Rude extruder spinner for Monoprice Mini which is an immature way to keep an eye on the extruder movement on my Monoprice Select Mini.
Well, I got a Prusa i3 Mk3, and saw their blog post talking about several of the extruder visualizers available if you're willing to add an 8x3mm magnet to the print with some superglue. I like the idea, and printed out the round one and the windmill, but I knew what I had to do.
Did an image search for "middle finger" and found one that I liked (you can have it in a lapel pin if you want) and traced it in SketchUp. Only really used the listed source for the outer diameter and thicknesses, otherwise my part is completely drawn from scratch.
One caveat - the original design has a slightly raised center section which keeps it clear of the motor housing. Mine has that but it's kinda on the wrong side, you won't get the outline of the fingers if you put the magnet on the top. Installing the magnet on the back, the part will slightly drag on the face of the motor and stop rotating occasionally. The easy fix is to add another 8x3mm magnet to space it out farther, or just order up 8x4mm magnets if you don't already have magnets on hand.
Obligatory video of it in action: https://www.youtube.com/watch?v=4eH74JOT0pk
Update: I printed a three-color version and the center hub didn't come out orange enough, so I added a ball to the end, which came out nicely. The model is non-manifold and I couldn't easily clean it up in MeshMixer, but it sliced okay in Cura at least, so I'm leaving it as is - which is damaged.
Doesn't have to be 100 micron layer height, that's just what I had loaded from a previous print. Am considering a multi-color version with black for the outline, will post that as a make. Used PrusaControl for slicing as that is really easy and works quite well.