I have a very nice set of about 30 different brightly colored gel pens that I would like to use with Silhouette.The inner diameter of the adapter closely matches the diameter of those pens. You will have to adjust that based on what you have. You will also need M5 bolt with M5 nut nut to act as a set screw, or adjust the nut well size to match different screw/nut.
The dimensions in the files may be wrong -- I had to fudge them so my printer produces desired output.
(Edit: combined knob and adapter in one file, exposed customiztion parameters for OpenJsCad)
1) Print the pen adapter.
2) Slide M5 nut into the nut well.
3) Make sure that it accepts M5 screw in its final position.
4) Print the knob part.
5) Use a vice to press fit the knob over the head of round-headed M5 screw.
6) Insert the pen so that ball point sticks out about 0.5 mm. Fix it in that position with a screw.
7) Insert the adapter into Cameo and lock it.
1) go to http://joostn.github.io/OpenJsCad/
2) paste the contents of silhouette-pen-adapter.jscad into "Playground" text area
3) Click "Update" button
-- OpenJsCad should render the adapter and create a form with a list of adjustment parameters.
4) adjust the parameters and hit "Update" button
5) generate save STL
6) optionally select "knob" from a drop down menu
7) generate and save STL for knob.