I've lost countless license plates through the years from boat trailers. They are notorious for dragging that "little plastic holder" on the ramp and snapping them, but only on the way out after you've checked to make sure it's still there.....
My solution was to buy a truck "mudflap" from Tractor Supply, trace the outline of the "plastic thingy" onto the 1/4" rubber flap and using my band saw to cut it out. After drilling the mounting holes for the plate, I've never lost another one.
You might want to consider an upgrade mod ?
I would post a picture, but the boat is "at home in the pole barn" and I'm in Galveston "enjoying the warm sunshine and cool Gulf water"