There are many formulas, most developed by trial and error and unlikely to be shared. Most of these will be based on polyester resin. I just got a catalog from Industrial Polymers
www.industrialpolymers.com and they list a urethane casting resin called 'Polywood' which would meet your needs. They seem to be an excellent company and their catalog gives some of the best advice I have seen for handling these materials.
ps; cast squids and an octupus in the catalog too.