I wouldn't be surprised at all if you found the whole 1/4 had been brazed on. It was a common method and I've done dozens of them- and none came off, whatsoever. The only reason you're seeing bubbling there is because the guy who brazed it didn't do it properly and completely. The factories used braze in their 1/4-roof seams. If it IS a patch, just make one out of 2 pieces and once you've got the inner and outer shaped, mig 'em together and on. If you haven't done that kind of thing before, cardboard is your friend. Good luck.