David:
Take it to a local weld shop, if you don't have a welder or access to one. I don't recommend it, but you could infill it with fiberglass mesh and resin. That stuff bridges well and can be sanded smooth and feathered in to be unnoticeable. With a little rattle can paint, you have infilled the hole.
I have also seen these hole used for a frenched antennae, but that requires a welder and some welding skills.
Good luck with your project.
Tony
