Hole plugging...

If you have the room, drop in a Scotty 244 flush mount.  Then the hole will be repurposed.

That's an interesting idea, but I don't think I really need another mount in that location. I'd. Probably rather avoid drilling MORE holes when I really need fewer...

Thanks for the suggestion!

Scotty makes one that only requires the single hole & a nut on the backside.  It's round.

Scotty makes one that only requires the single hole & a nut on the backside.  It's round.

The Scotty mount 444 requires a 1 1/4" hole.

If you want that hole permanently sealed, plastic welding is the way to go. Or you could use a 1" drain plug.

You might consider stainless fender washers: 1 that is exactly the diameter of the hole (centered) and 2 that are slightly larger (top and bottom). With a little goop and a short bolt, it'll never leak, and cost is negligible.

I actually tried this and it worked great. I'd post a picture, but I ended up repurposing the hole...

Ace Hardware or Orchard Supply has various size plastic snap in plugs for less than a couple bucks.  Usually in the little bins near the screws, washers, & specialty little gadgets.  Add a little GOOP & done.