This is non invasive, gravity pen holder for original Prusa i3 printer. Pen is mounted on extruder stepper motor. Holder is designed for Edding 0.3mm pen for drawing PCBs but is slightly modifiable in OpenSCAD file.
Pen holder in action can be seen on YouTube video https://www.youtube.com/watch?v=Mq9V-7Mj8eU
Printed from transparent PET.
Holder consist of three parts. You need some M3 screews and nuts.