.020" radius is somewhat better than a square corner. You could drill the corners with a 1mm drill. The followup finish operations then don't have to cope with excessive material in the corners, which never wants to come out very well because of the collision of chips coming from two directions.
You can then mill up to the drilled corners, probably with a 3/32" endmill. You'll have some slight scallops to remove. I would consider making a punch to finish shaving the corner areas. Make a pilot on the punch that matches the machined hole, and clears the scallops, followed by the finish rectangle that performs the final shaving. Press the punch through with an arbor press. This punch would basically serve as a single tooth broach. You could get fancier and add an additional stage or two to the punch to make incremental cutting possible.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)