Although this is an old thread, I have a suggestion for making a ball vise; use a jeweler's cast iron pitch bowl filled with lead, and topped off with a plate to mount adjustable vise of choice.
These bowl are sold in two or three weights and diameters, and the cost is low enough to buy two or three for different set ups and still be light years ahead of buying a nice GRS, one of which would be using pitch to hold odd shapes.
Since the bowls are cast, the outside is a little rough. That is not necessarily a bad thing for controlling the vise movement. A smooth surface is easy enough to make with paint polished and waxed without sweating the rough surface, or by using body filler under the paint to get a dead smooth surface.
Another useful parts holder -
http://www.gesswein.com/c-185-engraving-supplies.aspxThis is just one seller. I also have a half spherical steel bowl. These can be bought from online vendors, but they are a little more costly.