Using the template made from masking tape before, I cut out a piece of metal plat and tried to figure out how to get it to fit the shape, it's not obvious from the photos but there are quite a few curves and bends to be accounted for.
Ultimately I cut out a sliver of the metal in the shape of the curve of the door, based on lines drawn on the masking tape, bent the metal and welded the curve in place.
I then offered that up to the door frame and used the back of a ball peen hammer to shape the metal, stopping every now and then to add a tack to keep the shape in place. I kept hammering, then tacking until it felt flush against the old metal.
I ground off the tacks, and took the part off, before cutting out the worst of the rust.
With a let of fitting and cutting and grinding I managed to get the new piece to sit more or less in the hole. I still have more rust to tackle before I can actually weld anything back in.
