I use RTV all the time and make medallions, parts, etc for folks. In our Local SCA group I cast a large portion of the local awards. It is really simple to work with. I have not tried "thermosetting plastic" (researching now...hehe) but the Pewter (also known as Silver solder) is pretty easy to work with. I use an old electric 1 cup melter (for making bullets) that the electronics died on years ago. we just set it on the gas stove to heat it up, works great.
If I am only casting a single of something or making something in bronze (higher melting point), I use Delft clay and sandcast it.