This pen holder uses rubber bands to both grip the pen and to apply slight downward pressure. It mounts to my FLSUN i3, but I will include the link to my TinkerCad design so you may adjust the mount to your liking.
I saw quite a few other pen holders out there, but I didn't want to use screws to hold the pen and I wanted to be able to grip a wide variety of drawing implements. The contact with the pen is either a V shape, or a parabolic curve, so it should accommodate a wide variety of pen sizes -even a sharpie should work.
I used the same software and process that you would use to create a path in a gcode file for laser engraver attachments. Basically, you start Inkscape and load a picture, then you trace the path of the image and generate the gcode using the J Tech Photonics Laser Tool.
See more on this process at: https://jtechphotonics.com/?page_id=2012