I bought a cookie sheet from the dollar store. I cut a piece that was about half an inch bigger all around the hole. Then I riveted the piece I cut out to the cookie sheet. I used RTV sealant to seal the cracks. That way the original metal fills the hole, and the cookie sheet holds it in place.

If I were to do it again, I would have only cut 3 1/2 sides and made a hatch.

I (had someone) use a pneumatic cutting wheel.

 

I will look into that. Thanks.

 

FWIW, I bought a piece of sheet metal from Lowe's and some self-tapping metal screws. Trimmed the metal and placed it over the hole with some RTV along the edge, and used my drill to run the screws right in.