My new shaft just came in from S&S Propeller (in Flushing NY). I had given them my coupling and prop with the old shaft so they could face and fit the coupling, and lap-fit the prop. They added a tapped hole to the coupling and fit a bolt to line up with a recess they drilled into the shaft. With this in place it's not going anywhere.

The new bolt is the one with the square head, in this picture.