I have made repairs like this by shaping the relevant section over a piece of steel that I cut and ground to the exact profile.
(much like Robins pics)
I then Mig welded those sections in
A couple of professional bodymen looked at it, and they were very surprised at the high quality, as it was not detectable as a repair, still perfect more than 10yrs later
I did find it fairly easy, and the perfect place to improve ones skills in metal shaping and mig welding
In my opinion, that is not a good area to use bondo or other similar materials, as it is somewhat structural