coin disappear by capturing it with a magnet. I want to make a coin with
a large enough hollow for a supermagnet disc. Has anyone a procedure they
can walk me through?

Use two coins. face to 1/2 thickness, bore out 1/2 of the pocket. Repeat on
other coin. assemble and bond the three parts. For holding the coin, I'd
suggest softjaws, very wide for a lot of holding area.

Karl