There is an aluminum plate encapsulated in the fiberglass that was drilled and taped for the bolts. Simply tighten but if you want to put a backing plate on anyway, do so with a one piece drilled for all three bolts to distribute the load using lock nuts. If you want to ensure that the lock nuts will stay put as well, add lock washers as well after you have tightened those bolts first.
When the rudder is up any, there is more force on those plates. I was very careful in going slow with that rudder up when motoring onto the trailer. I hope this answers your questions. Sorry for the delay but I started on modeling the basement.
In retirement, you are suppose to downsize but I went the opposite direction with the new house due to grandchildren now and we are opening our home to Campus Crusades and others which is a part of our soon to be ministry.