Tom, I cut pieces of non- water absorbing foam , covered the side toward the street with black Gorilla tape and stuffed them in the opening. I then cut a piece of .040 aluminum the size need to cover the inside and glued and riveted it on.
Works like a champ
Vaughan & Tracy 2013 RW 36RL, 2013 F350 DRW CC, 2016 Focus. MorRyde IS, MorRyde pinbox, disc brakes, Comfort Ride Hitch. "Life is to be embraced, Grab it with both arms ! "