Here is how we made a replacement rudder for our 1971 Ericson 32 when the original began to weep rust out of a crack. Instead of waiting for a rudder failure, we had a local machine shop weld a heavy schedule 80 SS pipe with a series of 1/8" SS plates, which we infilled with foam and covered with glass. To add some additional strength, I put a layer of carbon fiber on the leading edge.
https://www.youtube.com/watch?v=uOAX4hHxUR4