""""""I can make it with a mill or 3d printer""""""

Not really. In the real world, there would either be "some amount of material there" or "no material there and some amount of gap"....

In the real world, there is no such thing as "just a surface"...

So, you could either cut a very small gap in that corner, or fill in the corner with a very small volume. Both could be not really noticable to the eye