This is my design for a pen holder attachment for my DIY CNC machine (based on the Openbuilds Acro). Inspiration for this design came from the Parametric pen holder for CNC by yannpom. Design considerations included using screws and heat set inserts for holding the pen tightly. I also designed the holder to be printed with the arm flat on the print bed, to allow for easier printing.

The pen holder attachment is mounted to a mini linear rail z-axis using a mounting plate that I also custom designed. I have included Fusion 360 and STEP files so that the design can be modified for the mounting plate on your CNC.

Here is a video of the pen plotter in action, drawing Serenity (from Firefly).