To hold a standard Sharpie Fine Point Pen without need for screws and to be able to cap the pen without taking it out of the gondola.
Use it in conjunction with i-make-robots Drawbot:
The tip of the pen should stick out of the bottom just 5 or 6 mm, so the gondola does not "swing" before moving the pen. The pen holder has a stop on the inside at the right depth. The two side "arms" hold it in place.
The V shape at the bottom can be useful for extra weight (in case your steppers can take it).
SketchUp file is attached for you editing pleasure.
Pictures are from a previous version - now the pen sits a bit further down for better contact in more vertical setups.