Originally Posted by Beezer
fly cutter is one way to make a big hole,
if you have time and a drill.... draw the circle, drill holes along the circle that just barely touch & file smooth.
as for the rectangle... drill a hole in each corner & use the cutoff wheel to run between the holes. clamp a guide strip if need be, and don't try to cut all the way through in the first pass. the thicker the metal, the more passes
Fly cutter on sheet, but on plate you can drill the periphery and file, you could speed that up a bit by drilling say 8 holes, connecting the dots with a dremel, or even better, a jig saw and metal cutting blade (or 6). Slow and steady with the jig, but it would move through the metal a bit faster than a dremel.