copper back up
Copper back up will absorb the heat on the thinner material. I got a couple pieces of good copper flat stock at the metal recycler about 3/16 thick, to clamp under the weld area. also a few pieces of 1 in copper water pipe to hammer to match contoured places. I also have a can of heat absorbing putty to shape and stick near the weld area. I the old days it was made with asbestos, and you could mix water back into it, the new stuff is asbestos free, but is harder to remix the water back into it.