This is a little decorative thing I modeled to show what your extruder motor is doing. This one uses a small 6.2mm diameter magnet and sticks to the stepper using the magnet. It works great on a Prusa where the motor faces forward, and doesn't obstruct any other printer movements. I'm sure it would work on other printers, but I'm not sure which. I was inspired by this post from Prusa's website:
Keep in mind this print is very small when slicing and printing, and you will need to rotate it 90 degrees to print properly.
I just realized it also works great as a cool white board magnet, or magnet for any other surface!