This is the front part of the CNC pen plotter shown in the pictures. Please refer to the YouTube video for seeing the design in action.

It uses a 9g Servo to move a pen up and down. Pen is fixed with two cable ties. In the middle I used aluminium tubing (OD 6mm, ID 4mm). All the parts are aligned with one piece M3 threaded rod, components hold on positions with a couple of M3 nuts. The spring keeps the pen pushed to the paper in idle mode. Attachment of Servo and of the assembly itself to the CNC plotter is done with some hot glue.

Designed in Onshape; public link to CAD - source files attached: https://cad.onshape.com/document-redirect.html?d=/documents/9158d083cabd84b78d2e0afb/w/46fec7ef818f30d1c785996b

See fully assembled CNC - Plotter in action: https://www.youtube.com/watch?v=xJS5MQ620vE

Goes together with this design: https://www.thingiverse.com/thing:2746214