VESA 75 mm with 24.6 mm ball. This will work with RAM or 67 Designs mounts that use a 1" ball. I have considered covering the ball with a thin heatshrink to give it some compliance, but haven't tried it yet.
In PrusaSlicer, I added a cylinder modifier slightly bigger than the neck, and added a 100% concentric infill to strengthen the weak point. I printed the rest with 20% infill.
This is my first published model. I hope to provide more!
The author hasn't provided the model origin yet.