KENAT, some tricks are possible, like:
Drill 4 holes at the corners, first.
Then plunge a big endmill right down the center to remove most of the mass.
Then change to a somewhat smaller endmill, mill a square around the round hole,
then change to progressively smaller endmills and mill progressively larger endmills,
and maybe finish by pushing a one-step broach down the hole to smooth the square.
OR, plunge an array of circular holes,
then plunge another array of smaller circular holes in the interstices,
and recurse until you can knock out the remaining stuff with a chisel or a small endmill.
In really old toolsets, you will find a drilled hole slightly larger than the square, right down the center of the square, straight broached sides, and tightly curled chips left behind by the broach, still packed into the bottom of the hole, fifty or more years later.
It's probably best to just specify a minimum depth of the square hole, a maximum corner radius or a note about a maximum relief, and a minimum solid thickness of the web beyond the end of the square cavity.
I tend to get sucked into super- detailing a solid model, making believe I actually know the best way to make what I want, until some old machinist sees my print, spits on it, and then shows me a better way. ... or doesn't show me, but just asks what features I really need, and goes ahead and makes a better part for less investment than I could ever imagine.
Mike Halloran
Pembroke Pines, FL, USA