have you ever considered fibre glassing?.. I never used this stuff before but I went to canadian tire and bought a boat repair kit and used that to make an Ipad holder for my truck … it turned out really good and I started with a cardboard(cereal box) design and glassed over it… used some tiger hair bondo for marine work( which is waterproof not like regular filler) just to fill in small imperfections… I must say for the first time with this stuff it wasn’t hard at all and cheap $30 for the kit.. this should fix your issue no problem I would suggest gluing it with super glue back together then glassing the underside that wont show for strength and filling the top side with some plastic filler then a really fine sandpaper to smooth it out and finish it with some paint it will be as good as new.