I've seen other pen holders and I didn't like the spring mechanisms that presses the pen down to the drawing area. (rubber bands, spring in the pen, …)

My holder uses meander springs that are printed with the part. This spring can be compressed about 2mm.

The holder can be mounted with the screw that is used for the print head housing. Up to three M4 screws are used to fix the pen in the holder. (M4 nuts are also required)

Setup of the pen holder:

1. I'm placing a 3mm spacer to the screw that usually switches the z-axis end switch. Thus the printer stops at 3mm before the nozzle of the 3d-printer touches printing bed. This is now the zero z position for the pen.
2. I'm executing a tiny script that drives to the new zero and then moves the z-axis 10mm down.
3. Mount the pen to the holder and use the stairs for height adjustments. The stair has different heights from 8mm to 10mm in 0.2mm steps. If you place the pen tip to 9.6mm then the drawing will happen at -0.4mm. (or the spring will be compressed the 0.4mm.)

Drawing:

I've used inkscape to generate g-code for printing. One g-code gernerator was already installed.

I've watched this youtube video to learn the basics about plotting: 

To print on thicker materials (e.g. 10 thick wood) I've created another spacer (10+3mm) for the z-axis end switch. After the first setup, I now can use different spacers for different material thicknesses.

Enjoy.