ArtMetal

talk to a sheet metal fabricator with a cnc turret punch
if they have the od punche, they can punch/or nibble out the rectangular hole and then knock out a clean round blank with the hole already in it. make you 20 of them for the same price as one.

or saw and file to fit.

get Andy's yet to be finished tome on filing. (go andy)

the Gene Olson
Sculptor
Elk River, MN