I agree use fiberglass mat.. The way I would do it is first grind the backside of the fender edge to roughen it & expose some matting to promote a good bond... Then use thin cardboard like a cereal box is made of...First I use clear packing tape to seal the cardboard... Then lay it over the front face of the fender & mark the shape on the cardboard, cut along the line then tape the cardboard so it's edge is butted to the fender edge, this will create a small step the thickness of the cardboard in the soon to be laid new fiberglass, I'll explain why later.. Now working from the backside, cut a 1.5" wide strip of fiberglass mat, not cloth, mat... mix up some resin & activator drip the strip of fiberglass cloth in the resin & brush a layer of resin on the cardboard mold & the backside of the fender.. Then lay the fiberglass cloth & brush it into place, let it set up a few hours then apply a second layer of cloth to the back side, same deal soak the cloth in resin & brush some on the area before applying the cloth.... let it set up for a day, strip the cardboard then brush a good heavy layer of resin on the front side to fill the void the cardboard left.. This method makes sure no cloth protrudes from the finished fender surface.. You can build the fiberglass resin up to the point no plastic filler is needed..