I filled with 3m HS repair filler (a stronger and more structural bondo). scuff the inside well and apply bondo from inside with a generous overlap on skin to create a secure plug. Then shape the outside with sandpaper, blending into the skin surface. You could also use epoxy/flox. If you don't care about the cosemetics, you could fill it with silicone after paint to stop the air intrusion.
Larry