This is a pen holder for a Wanhao i3 duplicator (plus) or Monoprice Maker Select (V2/V2.1/plus).

The two holes are meant to fit onto the layer cooling fan mount on the X carriage using M3 bolts and nuts. I used 2 12mm M3 bolts and nuts to secure the part to the fan mount (pictured).

An additional M3 bolt-nut set is used to secure the pen in place. The hole for the pen is sized slightly above 10mm. The pen securing hole can be used to resize the hole to allow for slightly larger or slightly smaller pens/markers to fit.

Approx. -17mm X offset from nozzle
Approx. -42mm Y offset from nozzle

Simple instructions for converting your printer into a plotter can be found here: https://www.youtube.com/watch?v=sXb9NDqn1Mo
(only simple changes to Cura slicer profile required)