The message . com
from "John Laird" contains these words:

Corner radius ? The cutout is not a full rectangle and a 20mm radius
on the corners sounds about right. You would measure out the enclosing
rectangle, though, and cut a smooth quarter turn from 20mm in, on both
sides.

I'd use a 40mm holesaw on the corners and join up with straight lines.
Remembering to set the hole centres in by 20mm each way, of course.

--
Skipweasel
Pay no attention to that man behind the curtain.