While not as good or permanent as welding, JB Weld works well also.
On that metal, as long as your machine is set up right (amperage), get a copper welding spoon, or just take a piece of copper water pipe, hammer one end flat, put a crook in it, and hold it against the back. for that size of hole, should take no time to weld and very little heat transfer.
You can also get a product that will absorb the heat away from the metal. It is reusable and looks a lot like modeling clay. use a flap disk to smooth off the weld and get it.
