I'll try to take a picture this weekend, but I took the simplest route.
I cut a 1/2 inch thick piece of plywood so it was 3 inches or so wider and taller than the hole, then painted the outside white.
I then put glued and screwed some chunks of wood on the inside of the piece of plywood, to position it squarely in the framed hole.
Then....I drilled a hole in the center of the plywood, and stuck a 9 inch long carriage bolt through the hole, with a large fender washer.
On the inside of the trailer door, I have a chunk of 2x4, with a hole drilled in the center, that is a little longer than the inside framing for the hole is wide.
WHen I want to plug up the hole, I slap the plywood cover on, and use a wing nut and large spring washer to snug the chunk of 2x4 I'll slide over the bolt, against the framed opening.
It works. I did put some tape backed weather stripping on the underside edge of the plywood to seal the hole up a little.
It takes maybe 2-3 minutes to put the cover in place, and it didn't take any expensive hardware.
The head of the bolt is rounded, and kind of flush with the outside, so there's really no easy way for a crook to get inside by trying to loosen the bolt from the outside.
From a distance, it doesn't look too bad, and may just leave it this way.
I'll try to take a picture this weekend, but the cover is essentially clamped in place with one large wing nut.
I put about 350 miles on the trailer, and the cover stays in place, doesn't seem to loosen up.