I saw a video by Cerven Solutions on youtube demonstrating using a sharpie to draw the bounds of a CNC milling machine. They mounted the sharpie in an ER20 holder with a 3D printed collet. I thought, "I want that" so I made it.
I don't have an ER20 holder handy, so for now this is untested. I'll update once I've tested and adjusted if needed.