If you are still considering extending what you have I would suggest adding a section mid rudder instead of adding to the tip. Taper the blades glass on both ends and build up the thickness to the outer side getting wider with each layer of cloth. Jake has a lot of pictures of glass work on his home page and might be able to give you more information on the process. There are a few repairs shown at www.epoxyworks.com if your not sure about how to go about always consult a boatyard for help.