The drill and file method you mentioned is ok but very slow. You could use a angle grinder instead of the file but be super careful , wear face protection and rotate the socket slowly. The socket will want to fly off so it needs to be firmly jammed onto the extension.
Buy a little lathe, every vehicle restorer needs one !
Mount the socket directly in the drill Chuck and grind one half then turn it around grind down the other half.
https://m.youtube.com/watch?v=iPa0jFlFW70