Grant Erwin wrote:

How'd you mill a pocket with square corners? - GWE

Drill or bore a round hole slightly larger than the across-the-flats
distance. Then broach. I've done this for both square and hex. I made
the square broaches from OCS and used pieces of old Allen wrenches for
the hexes. Machined (carbide) a short length at the end round and of a
good sliding fit to the round hole. This acts as a pilot to get the
broach correctly lined up.

Ted