Yeah, flatten a piece of 3/4" or 1" copper tubing and wedge it in place tightly under the fin
so it bridges the missing area. Now when you weld in the area the filler won't just fall out.
The molten aluminum will rest on the copper and solidify as it cools. After it cools, remove the
wedges and lightly tap the copper to separate it from the aluminum. Grind into shape and you're
done. The aluminum will not stick to the copper. this also works with mig and tig.