You can get steel 'spheres' from places like FJ Brindley, Biggest they list is 150mm diam. Ought to be adequate for a small press. Just need to drill it then, of modify the carrier somehow.
Another option may be to use some tube, capped of and a smaller bit welded through, and filled with shot...... or a series of thick discs, stitch welded together. A local fab place might have something to do such with.
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