Nearly there with the flex coupler!
I had the rubrail rubber off of my boat, cleaned it up and replaced it. If your rubrail is the same as mine, then again - a pretty easy deal. Unbolt the two metal caps at the stern ends of the rubrail (each should have two bolts through the hull
joint) and the rubrail should just pull right off. It just kind of pinches the hull
joint all the way. Reverse the procedure to re install it. Should take you a half of an hour total. Make sure to clean and inspect the hull to deck joint while you are there.
Sounds like a great score there. Last I looked, a new one is about $300.