what i would do if I were you is go down to Lowes/Home Depot and get you a small section of Water Heater exhaust pipe (not sure exactly the correct name, but its a thin sheet metal that has an overlap to snap the flat piece of metal into a circle). Just get the right diameter, then trim it to your offset and you should be good to go.
Like Vano said cut the hole with a torch and itd be easier to use a die grinder to smooth it out, that's what we do at work and you'd think it was drilled
