@NLXTACY ^^I like the sheet metal and seam sealer.
Or, if you spend a few hours on the internet searching, you may
be able to find a correct size/shape oval rubber or plastic plug that fits.
(maybe another Wits-End project?? I want a royalty for each part sold )
For a quick fix for something like this I've used heavy duty HVAC aluminum tape
(Home Depot, Lowe's), two (or more) sheets, one from above and one from underneath after cleaning off (degreasing) all surfaces well. It should last for awhile in that location, but won't remain waterproof forever.