I've been trying to improve the center mount so that it prevents rotation. The plan was to add a raised tab to the ball, then fit that tab into a slotted socket (constraining rotational movement down to one axis) then fit that slotted socket into another slotted socket (which would add one more axis of motion). The result was supposed to be movement in two axis without rotation.
This seemed easy enough to model in SketchUp for printing on the Makerbot, and if it worked, I'd send it off to shapeways. Well SketchUp is really the wrong tool. Whenever you make hemispheres with volume (5mm thick walls) and try to cut notches through it, you get messy messy results. I finally got something to work by downloading this person's work, hand editing out a notch, then making a copy that was 5mm larger all around, and sewing them together to create a closed volume. It works but is really lame. Results below.
So guess I'll try this in Autodesk Inventor which I've completely forgotten how to use. And we're moving in two weeks which should subtract lots of spare time. Oh well. Stiff upper lip and all that.