Don't use more plaster. It would have to be solid to not break, and even then is still fragile. Get some spray adhesive. Spray and cover with strips of newspaper. After a couple spray-adhered layers, papier mache. Half white glue, half water, quickly dip newpaper strips and used your fingers to get off excess (stick between your fore and middle fingers and pull), and layer layer layer. Only a couple layers at a time, wait for them to dry, then add more. Alternate directions of the layers for the strongest form. Paper is surprisingly strong.