I've been trying to find some plastic or rubber fin washers for my fin screws for ages, Bunnings and a couple of other hardware stores didn't sell them.
Tonight I managed to find some at my local bottle shop!

I will need to do some thorough testing but so far they look pretty good, the only down side is the cost. At $5.50 a washer they're pretty expensive, but I guess you have to pay for quality.
Just have to warn you that removing them from their packaging has some side effects and not recommended for children under 18!
I'll provide a full report after thorough testing.