After I explain why I say use body filler or weld and body filler, it will just continue the wizzing contest. I can not tell the extent of the, so called, rot. I need to see the bare metal, as does the poster. What I see does not seem to need a fabbed piece. It does not seem to be structural. Cosmetically, it will be hidden. It does not seem to be ready to break off. But not sure. It seems like it just needs smoothed so that the windshield will seal.
Body filler is a two part compound especially made for auto body metal smoothing. It mixes to a nice spreadable consistency, easy to apply and easy to sand. It will fill small holes. It is used by professional body persons worldwide. They don't use JB Weld for body work. JB Weld is not for that. Too thick, too messy, to globby, not smooth, harder to shape and sand. The real question is not for me, it's why use all these other ideas. Why not body filler? That is what it is for. There is nothing better or easier. It won't take much.
The choices, depending on what is found, are:
1. Body filler
2. Weld a little and body filler
3. Weld in a new piece and body filler
4. Solder or lead it
I will repeat, it's the OP's choice.
There are probably better words than "smoothing" but you know what I mean.
