I would not use a polyester casting resin. Use a casting epoxy with 1/32″ – 1/16″ milled fiber or chopped strands. 5% to start would be a good guess.
The physicals of epoxy are far greater than polyesters. Another option would be a casting urethane. You could also add milled fibers or chopped strands to urethanes.
The milled fibers or chopped strands act like re-bar in concrete and add a massive amount of strength.
The hard part will be the molds. They need to be super smooth, and have enough draft so the finished part will come out easily. Matched metal molds would be a great option, but the cost of machining and polishing would probably not be justified.
ABS injection molded with 10% chopped strands would have great strength, but once again the metal injection mold price is very high.
If you have any more questions regarding epoxy, urethanes or casting resins just ask. I happen to be a certified composite technician..