Had the same problem. Made up a repair patch which I stuck in with panel bonding adhesive (two part epoxy adhesive used by manufacturers and body shops for quick effective replacement of panels including. Google "3M Panel Bonding Adhesive" and look at some of the YouTube clips that come up. It's amazing stuff!

Here's the patch panel I made. spot welded flanges about 25 mm wide along the two straight edges, spread panel bonding adhesive on them then slipped the under the doorskin and wedged them into place until the adhesive cured.

Final step was a skim of body filler and job's done!

(Of course the inside of the door was thoroughly de-rusted and plenty of rustproof paint was applied before the patch panel was glued in place. The inside of the door was also cleaned thoroughly. Panel bonding adhesive is also a sealant and I expect this repair to outlast me!)