It was a smaller rudder being on a 22' boat but some principles are the same. My armature was flat stock welded to the rudder shaft, all bronze. In the areas of the flat stock I cleaned it well with acetone then laid on strips of 'glass to the depth of the foam so that there would be solid contact with the rudder skin from the armature.

I only used the foam, Airex, to fill in the voids, so to speak. After shaping it all with rasps, etc. I then skinned that over with 'glass and epoxy. I was mindful of rudders of foam wrapped on armatures in which the foam collapsed over time under the stresses of water and turning and all that.

After 28 years, I had a chance to see the boat out of the water, the rudder is still solid. If it leaked anywhere, it was not obvious to the nekkid (that's for Blye) eye. Boat was launched in 1980 as seen being raised for loading onto a flatbed in the pic. I was younger then.