Slip this on a Sharpie Fine Point pen (aka: a "normal" Sharpie) to use it in a standard pen plotter. Note that regular Sharpies are too long to fit in a carousel-style pen holder. I'll look into updating this for mini-Sharpies eventually.

This should be a snug friction fit, but tolerances are tight and printing it is tricky. There's only room for a single 0.4mm perimeter wall, and you'll need to print with support material. I printed with 0.2mm layers.

Note that the long end goes towards the tip.

Note: I made a parametric version of this adapter available here:

http://openjscad.org/#https://gist.githubusercontent.com/beardicus/d668c0f6b96be53d16dc/raw/plotter-pen-adapter.jscad