They do make snap-in metal hole plugs that small, one local Lowe's has
them. Anything that uses heat will be sure to fry your silkscreening.
Alternatively, you could find some sort of vintage fitting to fit the
hole, an earphone jack or outside antenna terminal comes to mind. Or,
just accept that you aren't ever going to make it look like new again
and live with the extra ventilation.

Stan