I second the Mig welding. Take an 8" length of 3/4" copper pipe and flatten about 4" of one end with a hammer. Clamp the flat end in a vise and bend the pipe in the middle so it's at about a 45 degree angle. When it's done, it should look like a spatula with the round end the handle and the flat end the blade. Either hold or have someone hold the blade on the back side of the hole and with the welder, start to fill in the hole. The weld will not stick to the copper and it allows you to fill a pretty good size hole. Then grind and prep as usual.
