Summary

I wanted to see if I could use the CTC printer as a plotter so I created a pen holder gadget based on the mounting system of the active cooling fan.

Since I liked the mounting system of the Active cooling Fan Duct it is kinda the same design and even uses the clip stls from that thing; check it out:
http://www.thingiverse.com/thing:537918

To facilitate different pen sizes the pen holder should be an exact fit for your pen to allow it to slide up and down. The elastic belt should provide a bit of a safety range so the tip of the pen won't be damaged. It might take some tinkering in OpenScad to get your size of pen.

Some interesting links to get it working:
For conversion of HPGL to Gcode: http://hpgl2gcode-4646.rhcloud.com/
For conversion of Gcode to x3g: http://www.thingiverse.com/thing:81425