I'm actually bloody proud of the result here, it was difficult and time consuming, but well worth the effort.
In the end, cutting the patch to fit accurately turned out to be the hardest part, hours of back and forth, plus plenty of swearing.
Bit of a shaky start with the welding too, but once I'd got into the rythm of it, I ended up with some of my best welds so far, and they cleaned up just fine with tthe flap wheel when all was said and done. I might even be getting the hang of it. No more rusty rear battery box at least.
One thing is for sure though, I need repair panels for the rear wheel arches. There's absolutely no way I'm going to be able to shape my own!
Here's the vid:



