I made an eyeball for the book from Hocus Pocus this Halloween. I created the model with some rough specs before the book was constructed. Based on those dimensions, I went with 15 kg/cm servos. These turned out to be overkill and a bit of a challenge to work with. The part designs are pretty ugly, driven by the constraints of 3D printing. Here’s the final result:

There’s just barely enough room to assemble the linkages inside the eyeball:

If I had the time to redesign it, I would put some bearings into the eyelid pivots. The Fusion 360 project is shared on AutoDesk Fusion Team.

My Arduino code for the eyeball is in the hocus-pocus-eyeball GitHub repo. It’s really important to set the servo limits; the plastic links are pretty delicate.

Here's a view of the mechanism before installation into the book: