Weld it up with a Mig and grind it flat is most effective and cheapest (if you have a welder). You can also braze a plug behind it (clean the flux out really well) and build it up with lead or bondo. Plain bondo in the hole without a backer will pop out easily.
The nickel plated plug or a carriage bolt with a painted head is ok if you don't mind a bit of unoriginality.
