I'll give the Monel rivets a try ......

And I'm aware of the 'cut' in the rudders from the s/steel plate, I did originally intend to drill a 1/2inch hole and insert an alloy rod into the hole and the tap a thread in the rod .....
The idea was to then add a small bolt to set the blade against.....

I haven't got to that yet, sailing it sort of took over

It's a modification I'll do over the winter break, along with replacing the main decks, there's a few other changes I'm going to try as well.