I'd guess a call to FWC and a couple bucks would get you a piece of scrap you could cut to cover the outside, then follow the goo and rivet approach as mentioned or if the idea of rivets showing is unacceptable then metal epoxy the patch on. Of course the repurpose idea is a good if you have a need for a repurposed hole.
The good thing about getting a piece of scrap is getting something that will match the contours in the metal and color. A guy may luck out at the hardware store and find siding that matches too. I don't think it's an FWC exclusive item.
