I would cut a round cover using 22ga, and rivet it in place, with the cover on the inside of the firewall. Use some 3M body caulk strips to seal it. Rivet from the engine bay so they will be flat. Paint it and no one will even notice it.

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"Tell it like it is or don't tell it at all" 1964 Falcon 2dr HT, 347V8 4V, dual exhaust, 5sp, pwr assist R&P steering, AM radio, heater with defrost and 2sp fan, electric windshield wipers, cigar lighter, dlx light group, bucket seats, console and full carpet.