Andy all you needs is one of those green circle templates from Staples or Office Depot.
Cut 2 at once crossing the tape for easy seperation. Cut on glass with a new #11 blade from nice sharp edges. Draw the reference lines on both to help with alignment on the wing/fuselage. Cover the hole with a piece of tape before you peel it up and it will stay nice and round while you position it.
The template is good to have around for rescribing too.
