I put some of the rivet backing plates (yeah, they look like aluminum washers to me, too) on the inside of the rudder tip. I am holding them in place with the Clecos and putting a dab of superglue on them to hold them in place. It doesn't need to be strong, just strong enough to hold them until the rivet is in place. I am hoping that I didn't also glue the Cleco into the rivet, but if I did, it should be a brittle enough joint I can just twist it off and reglue the washer, I mean backing plate, if I need to.

This should make sure that the rivets don't pull through the fiberglass, which is why future me will thank present me in a few days.