This holder clips into the center of a Root Robot to allow it to draw off-center. It is counterbalanced with an optional weight on the side opposite the marker.

https://rootrobotics.com/

It is designed for these paint markers, but can be adjusted for smaller markers.

https://www.amazon.com/Acrylic-Paint-Markers-Set-Permanent/dp/B07BSQJPGM/ref=sr_1_47?keywords=paint+marker&qid=1547826274&sr=8-47

Video: https://www.youtube.com/watch?v=h9NoDtw8VmM&feature=youtu.be