Use a very sharp drill bit and start with a pilot hole. You should plan on adding a rubber grommet to the hole before you pull the cord through and that grommet will hide the edge of the hole. Paint the freshly cut steel with some paint to keep it from rusting. Rust is more likely to blister the paint than is proper drilling. Drill your hole on a hot day with the safe thoroughly warmed up in the sun. Heat reduces the brittleness of the enamel and will also minimize chipping. You could also use a hair dryer or heat lamp to warm things up at the hole location. That's what auto body shops do for paintless dent repair.
