This particular rudder is built off of the original rudder for a ~20' Bayliner Buccaneer sailboat. The original had cracked and rotted pretty badly. The owner of the sailboat cut the top of the rudder off and made a wooden 'boot' to cap the rudder. However, it wasn't water sealed with fiberglass, and over time more and more moisture got in until it became so flimsy that it wasn't reliable.
While this instructable is specifically for this Bayliner sailboat with a tiller-style rudder, the instructions should be general enough for you to modify it to work for many sailboats. With that said, there are many many nuances to fiberglass/composite marine construction, so this type of build will require more research beyond what is covered here.
Step 1: Previous Rudder
In these photos you can see the extent of the damage.
The rudder was foam-core/fiberglass sandwich. Think of it as a Big Mac; the three buns of the Big Mac were layers of fiberglass, and the meat was the foam (the yellow stuff). The only difference was that the buns would have all been connected and fully enclose the meat.
First, I cut apart the rudder along its perimeter with an oscillating saw, so that I could use the pieces as templates for the build.
In the fifth image you are seeing a piece of balsa (I think) at the edge of the rudder where the mounting hardware was located so as to provide compressive stability for the tightened hardware.
In the last image, if you look at the top of the image you can see where the previous owner had chopped off the top of the rudder. There was a rudimentary wooden cap on that, so you can see how easy it would have been for water to get in.