I need to do this as well, for some optical mouse sensors.
Has anybody done interior milling? Otherwise I suppose I’ll have to do corner drills and cut it out myself somehow… doesn’t sound like fun. I can’t figure out what layer you’d put this on, since there’d be nothing on the silk, copper, or mask layers that would indicate it’s a cut-out and not just an marking or trace of some kind.
They expressly forbid slot holes and linear drill lines.