Harris. That is not of the 27X vintage. Sorry I confused the message.
Most Em Tillers I have seen use a through pin to hold them down on the head of the shaft. They tighten tighten against the shaft with a strap using friction to hold it agains the shaft. Some have a bar welded inside the shaft that they EM tiller slides into and over. The EM tiller post has a U cut in the post bottom to slide over the Bar. There are 3 elements of the design trying to hold the EM Tiller onto the shaft so you can turn the rudder.
Suspect you will be fashioning your own design. It will need to employ multiple ways to grab onto the Rudder shaft. I agree with
@Justin_NSA just adding a tenon into the shaft will not work for long. You might do that and add a strap around the rudder shaft to engage some friction as you tighten the EM tiller to the rudder post. It will also serve to keep the tiller tenon in the rudder shaft.