On my Forest River trailer, I had a similar situation. The hole was 4" around. That got me to my elbow
I got a 6" holesaw and made the hole bigger. I just didn't want the storage area open to the interior. I bought and stained a piece of 1/8" plywood. I cut into a solid circle to cover the hole with 2 protrusions with holes to screw it on (see inspiration picture).
The only real difference from the picture (besides the giant hole) was the bottom hole was a slot to allow it to swing in and tighten down.
Cannot find a picture of that mod, thus far. Not sure if I renamed it to something I cannot figure out the name of ...