For using with urethane resins (the two part stuff, like Alumilite or Smooth On), Silly Putty is a great temporary mold agent. Chemically, it's silicone based, which is what is recommended for urethane molds anyway, so you won't have problems with the resin eacting with it (sticking, not setting, releasing toxic fumes, etc). Some molding agents are NOT compatible with some resins, a lesson I've learned the hard way.