Chopped out the rot, put a 5mm alloy plate in behind the sound deadening to stop setting fire to anything and then tack welded in a repair panel.
Fully welded it in, cleaned up the welds, primed and seam sealed just to be sure.
One of the other things I’ve been cleaning up as I go along is this mess – looks like they drove through a puddle of black gunk. This was the starting point:
I’m about 50% the way there on that clean up…