Summary

I started with the revised fully printable eggbot here http://www.thingiverse.com/thing:20398
and found the servos on hand didn't fit well and had to work too hard.
There was also some backlash in the original pen design that I could only fix by crushing the pen so the pen holder has had some adjustments made.

Instructions

Make an eggbot attach these parts in place of the originals.
I have to rotate them in before slicing and the servo does need support.

My purpose was to remove as much backlash as I could and make a more accurate eggbot.
This is an "upgrade" to Glasswalkers eggbot pen arm.
A better term might be this is a specialization of the original.
The shaft mount is not included but I am still using the original.
The pen securing bolts and nuts are sized for m3x8 and are sized tight enough that a nut may not be needed.
The slider bolt slot is also m3.
The pen aperture will not fit a normal sharpie, but will fit the art type ink pens or a sharpie fine.
The servo mount is for a 9g hobby servo I got for 3$ at the rc store

Another spherebot eggbot armature