Summary

After trying out a couple of adapters here, I decided to venture out on my own. The simple version seemed best. I've included the two files for Fine Sharpie pens and Ultra Fine pens, but there are instructions on how to alter the F3D file for any pen you have, provided that the shoulder where the cap sits is at least 34mm from the pen tip and the widest diameter of the pen (in the area under the cap) is less than about 12mm. Let me know what pens you've adapted, and feel free to remix and post brand-specific versions! (Note: the Pilot G-2 adapter is meant for the top to be aligned with the back of the rubber grip since there's no cap shoulder on the pen).

Sharpie adapter can be seen in action in this 50 second reddit video.