Below is a nice looking Jam Chuck with slots cut in it to let It open and close. And a hose clamp to keep it closed.
When I took a class with Christian Burchard we cut patterns on spheres and hollowed them using Jam chucks.
Those were cylinders hollowed to hold the balls. No slots the balls were a tight fit and pushed or tapped in.
After hollowing the balls usually came out when the side of the Chuck was hit with a tool handle.
sometimes the chucks had to be split with a chisel to free the hollowed sphere
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