I also have a new favorite way of filling holes. First I have holes that need to be filled:
I then make some plugs for the holes, and then drill the hole out to a size that will fit the plug. Punching 16ga. was the hardest part of this operation.
I use a hammer and dolly to get it right where I want it, and fusion weld it.
Works great, and almost nothing to grind afterwards vs. how I did the holes in my firewall.