Unless you want it to come apart fairly easy, don't use kitty hair or bondo of any polyester resin product on your watercraft. When I rebuilt my SN, somebody had made several major repairs to the hell using polyester resin. using a scraper and hammer, I was able to peel most of the patches out intact within only a few minutes. There was practically no bond at all.
Epoxy resin, cabosil, chopped glass fibers.. mix to peanut butter consistancy. The chopped glass will add more strength to the "putty" sort of like kitty hair.
Use whatever form you wish to make the desired shape. Just remember you will add thickness with the glass. You might want to also think about a layer of kevlar on the outside of your additions. Kevlar is extremely abrasion resistant so will help to keep problems down.
Be warned... the cabosil will make the resin a PAIN to sand. Build your structure sand to basic shape with low grit paper and then fair it with epoxy resin mixed with micro balloons. This will make an epoxy equivalent of bondo that's light and easy to sand.
