The way I would do it (are ya'll ready for this big surprise?) would be to cut bits of styrene sheet or strip to fillt he large areas. You can sand these to the general shape, and cement in place. If you do it well, there should be only a very small gap, that you can fill with the tube type putty. That method is quick, and clean, and strong. But if you use entirely putty, an epoxy or other two-part would be best.