Clean and grind the edges. Cut a piece of sheet metal to fit leaving a 1/6" gap. I like painting the back side with weldable primer. Tack weld with a MIG gun...no continuous beads...smallest wire you have...test weld some pieces first. Slowly weld the piece on one tack at a time. Grind smooth. If you did it right minimal skin of Bondo. Primer. Paint.
-Mac