Thanks guys for all the suggestions and advice. I really appreciate
it.

In the end I tried the brown sugar method, because we have plenty of
it at the office coffee station. Long story short, it worked
excellently well. ZERO distortion of the heatsink fins. We went with
high speed and gentle feed.

I taped a cardboard form around the heatsink and tamped the brown
sugar down with 1/4" parallels. After the sugar was packed in, I
taped another cardboard piece to keep the sugar in the fins in case I
thumped it between my office and the machine shop.

We don't have a lathe at work, but I have one at home, so I ground
down a two-flute 5/8" endmill (based on Gunner's suggestion; improved
by others' suggestions) on the home lathe to get the precise O.D. we
need.

BTW, my company manufactures radio amps... the amp components all
mount to the heatsink, and the 'sink mounts into a rackmount chassis.
So our machine shop is somewhat limited. The 0.600" holes are for
coax end connectors, this is a very tight build so the holes had to be
relatively precise.

The purchasing department is going to wonder where all the brown sugar
went...