I just managed to construct and build a working counter weight for my shoulder rig.
A standard training weight is secured to the 15mm industry standard rig system using either 4 M5x16 wing-bolts or (if you live in imperial-hell) 2 1/4" wing bolts.
* the design on thingiverse
* the original rig on thingiverse
* my project notes in my personal wiki
* photo album of the shoulder rig -project