I put a dropout saver into a hole reamed out with a hand-held drill. It would be nice to get it precisely square and so forth, but (a) clamping the bike to the drill press can be an
interesting exercise, and (b) handheld works well enough.
As @
fietsbob noted, you'll be holding the dropout saver with a hex wrench while you screw the derailer on. If you've used a properly sized drill bit, the fit is snug enough you'll get minimal wiggle from the mounted derailer. Note it might be worth checking the hanger alignment before you put the derailer on.