I do not know where exactly he was born, but it was in CZ. Possibly he had to report the COLB at the consulate in Colon.
Look at the map.
Colón Hospital is clearly shown right at the very top of the peninsula, as far from the Canal Zone as one can get, and still be on dry land.
The base hospital wasn’t built until 6 years after McCain was born.
His BC says Panama.