I got a cut just like that 3 years ago on the dome of an Air Vent power fan.
DAMN, they make those things sharp-and for what reason ?- what would it cost? 3 cents to roll that lower edge over ?
you guys who won't make a cut back towards yourself wouldn't make it in the floral trade.
when my wife was a florist they made hundreds of cuts each day-everyone was a one handed cut with the blade pulled backwards and the thumb used as a stop.