Thanks Russell. From the pictures the Q-Max cutter looks much the same as the Wurth hole punch: I think you didn't mean to imply that stepped drills are basically the same as the Wurth tool, did you?
I've now ordered a Q-Max cutter. Still can't work out how to get two flats on the circle though....

What do you mean by " two flats " ? Should it be an oval ?

No, I didn't word it very well at all. Stepped drills are better than holesaws but neither will make as good a job as the Qmax cutter. They're one of my favourite tools.

No, not quite an oval. The USB socket (which fits flush with the surface) has a cylindrical threaded body, but the threaded body has two flat (unthreaded) surfaces cut out of it, one on each side, to prevent it from turning - but only if it goes into a hole of the right shape.

You couldn't cut the correct shape with any kind of circular tool. The hole would be like the shape you'd get if you turned the letter "U" upside down and put it on top of a "U" the right way up.

The flat surfaces are almost certainly unnecessary: you don't twist a USB plug when you put it in or take it out.