Using a mask like this, i.e., laid on the print at the time of exposure, is not all that hard. (Note that it's the print that's being masked, not the negative, which would need a lot finer registration.)
First, it's fairly easy to make the exact correct size mask from a reject print that was just made and use that as a template for the black paper mask (you can't easily use the print cut-out with fiber-base paper, since it curls). Second, in the example linked to, the printer wasn't using the mask for a significant amount of the overall exposure, just to hide the little lighthouse from the burn. That's not going to leave much of a halo, and likely none at all if it's registered halfway correctly.
I use this technique from time to time and it works well if you're careful. And, you can trash the prints that don't come out correctly.
Best,
Doremus