A little Knob for the Pen-Holder of the Spherebot / Egg-Bot:
https://www.thingiverse.com/thing:1683764
Requires a M3 Nut and a M3 Skrew (needed anyway for the Egg-Bot). The Knob needs some space, so I used a little longer skrew. But the 'original' M3 for the Egg-Bot also works fine for Sharpies.
Eggbot_M3_Nut_Knob_.stl | 129.8KB |