You could see some photos from Skribbles aka Skycode22 on his OpenOT, with the first teaser posted more than a month ago, but he just got back to me with the files which are publicly available now.
OpenOT is an ortholinear keyboard with a trackball integrated to the right side (the design could be mirrored for left-handed users).
The trackball mount used is called OpenMount, another open-source project of the author, and can be used for other trackball designs as well.
The OpenMount uses 10mmx4mmx5mm roller bearings, I found the larger size to impact smoothness and longevity in a positive way – Skribbles.
The encoder wheel is a unique orientation that is embedded into the case and trackball and requires a bit of technical knowhow to get it right.
The encoder wheel also needs a "center axle" that goes through and also connects inside the encoder wheel.
Personally I have been using 1.5mm steel baling wire that I cut and straighten to the right length, and then flattening out one end so that it "keys" into the encoder.