if I understand you well, I had the same leak from the rudder stock, after a crew lost the plastic protection on top of it. So, I replaced it with a new protection that I made. I will post a picture when I am back to France by late april.
I had the same problem with the white plastic collar broken and water leaking inside. I made a picture of it for you to better see how it is built : in grey, the deck, black the rubber cover and the oring, in blue the metal parts and yellow the white collar.
You need to replace the white plastic collar and the oring inside. Unfortunately you cannot find a replacement part. You have to make it yourself or have it made. What I did is on a calm day while the boat was in the water, I removed the white collar and brought it to a man who could build a new one during the day. I requested to have the ouside increased by a 1/10th of a milimeter to ensure it would stay in place. I also changed the oring and put everything back in place. No problem since.
je vois que tu as refait faire ta bague supérieure de safran en donnant l'original au tourneur.
On est plusieurs à vouloir se faire refaire des bagues. Aurais tu les dimensions ? Sinon, on va mesurer sur le bateau. Par contre, tu es le seul à avoir ce joint torique, ce qui est une bonne idée. On va continuer en ce sens.
Quel est la section du joint torique ?