Edison robots - simple pen holder

I came up with this simple pen holder as a quick and easy way of converting our Edison Robots into "draw-bots." I started with the basic LEGO brick here: https://www.tinkercad.com/things/1FSUvYnxIng-lego-brick#/ and just did the rest in Tinkercad.

The pen holder is sized and positioned for a Crayola fine-line marker.

After printing, simply snap the piece onto the back of Edison and slide the marker into place. In my testing so far, the part is a snug and secure fit on the Edison's Lego-size pegs. However, the part's pegs do not secure well to other Legos.