[Geof]

Here is a really crude suggestion...that does work.

Get a used 2 flute end mill the correct size, but not too used, all you want is the initial really razor sharp edge taken off.

Take it to an oilstone or if you don't have this some fine emery paper held flat on something solid and hone a tiny radius on the corners of the cutting edges; tiny means a few thousands of an inch, .010 to .015.

Drill your hole undersize like you would for reaming, you should be spotting it with a spot drill and you can take this deep enough to leave a small chamfer.

Now go in with your modified end mill almost like you would with a reamer, slowish rpm and fastish feed. The dulled and rounded cutting edges hold the end mill centered in the hole. There will be a little radius at the bottom of the hole from the dulled corners but that is unavoidable.