In stead of hunting for copper sheet I just took a 6" piece of copper tubing and flattened it out. Then I bent a 90* angle in it so I had a handle. Works perfectly.
I also have a few chunks of aluminum. That works just as well.
Just a tip. If you can, try to weld from the back side and have you copper on the finish side. If you hold it nice and tight to the metal the weld will be nice and flat and you'll have less finish work (grinding) to do.
Ted
