Check out these guys who who were given pink plaster casts after their shells were mauled by a nasty dog. The plaster casts protects them whilst a new shell grows back. After finding this RSPCA article I decided that taking a modroc cast from my tortosie was perfectly okay.
I got a pretty good result. Not sure if I am now going to use it as a mould for resin, plaster, latex etc.