Hi Bryan
I could find no way to stop water from entering between the nylon washer and the fiberglass lip of the rudder stock below where the emergency tiller would go. My boat is an 87 Mk I #692.
Asked my usually reliable boatyard manager to take a look and he threw up his hands.
Finally had my wife sew up a Sunbrella flap (8x10"?) which attaches with Velcro to the inside of the wood trim on the aft cockpit locker. It has a small pocket at the bottom for a weighted sash to keep it from blowing around. Not very elegant, since it doesn't match the rest of the canvas, but it hangs over the tiller fitting to prevent water entering.
I use the emergency tiller fitting to see where the rudder is before leaving the slip. In that case, the hatch is lifted, the flap is allowed to dangle inside the locker and stays out of the way after the hatch is closed.
The boat is eight hours away, or I'd include a picture. Adding "Rudder Stock Flap" to my Leaving the Boat Checklist has minimized, but not entirely eliminated, the times I've forgotten to take it out of the locker.
Hope this is helpful. Maybe there will be other respondents who have solved this issue absent hanging canvas over the offending source of the leak. There are some awfully clever folks out there.
Jack

Just a reminder.
At least once a year make sure you can remove the cap and then lube the threads. You do not want to be struggling with this in an emergency. Know where your tool is for opening the cap as well as the emergency tiller.
John Meyer
Currently in La Paz