I've made those dished plugs from flat steel about .080" thick. Cut/grind a circle of metal about 1/32" bigger than the recessed counterbore it needs to fit and dish it with a ball peen hammer and a socket in an arbor press until it's a close slip fit. Secure it with a firm tap to the center with a hammer.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!