When I installed the license plate holder (above), I had two holes from the old unit.
I used olympic rivets. The rivets leave a long tongue like the one above when it breaks off.
You cut off the tongue with a wire cutter.
And … grind it down with a Dremel. It looks just like the other rivets on the trailer now. Regular rivets have the hole in the middle that lets water through. This is why you need to use olympic rivets on the exterior. There is a rivet shaver available to grind the tongue down but it is pricey.