Hopefully this STL saves someone some time. I had to iterate on this about three times. If you don't have a 3d printer then let me know and we can work something out where you can acquire this mod.
Omnibot 2000 Neck servo Adapter www.thingiverse.com/thing:474444
The disc is stationary and also plugs the hole. The servo screws into that disc too. Looks like the neck screws into the servo spline.