After cutting the center 1 1/2" hole (cut under-sized with a 1 3/8" holesaw 'cause a 1 1/2" holesaw makes a larger than 1 1/2" hole) I spin the plate in a vice and very carefully file it, spinning it an 1/8th of a turn at a time... round and round and round. On DeltaCAD I drew successive holes up to the 1 1/2" and one past it (clearly marked with a dotted line instead of a solid line.) This allowed me to ease up on the hole and keep it circular.
