If you have easy access to both sides clean all the rust off, slightly dimple the metal in, close the hole up from the back side with some fiberglass cloth, bondo the outer side, sand and paint. Then get some spray undercoating and cover the glass patch.
This should hold you for quite some time, but don't be surprised if you don't find something similar somewhere else. Unless this part was defective allowing the rust there's a good chance it's endemic to the vehicle and more rust will show up down the line.
