I made it on my Royal Caroline. Originally I think they made it from canvas and probably it was tarred. On the model it has to be made as a cone first, the small dia will be around the rudder, the big one will be around the cutout in the hull. It will be pulled around the rudder first and fixed to it upside down. It can be nailed to the rudder but it is easier to twist 2-3 runs of thin line around it, then fixed with some glue, it won't be seen anyhow. Then the rudder goes into position, put onto the pintles. Then the canvas cone is pulled up and nailed around the perimeter of the cutout in the hull. Easier to do than trying to explain...
János