Did a couple more scribbles before moving over to Fusion to draw up the parts:
The pen will be secured in the center, inserted into the top two parallel rods that will act as a rail system for the pen holder to move back and forth, which will be controlled with a motor and a gear. The bottom of the pen holder will have teeth for the gear to attach to and move.
A preliminary drawing, trying to wrap my brain around the form I want this to take:
