ShapeLock Design Plastic, Maker Shed, $14.95
This stuff is a low-melting thermoplastic called polycaprolactone. Supplied in pellet form, it can be melted in hot water, or in the microwave, at 160° F (71° C), then safely molded by hand, cooling to a rigid, opaque, machineable solid at room temperature. It is reusable, and can be reheated and reformed as often as you please. If you have a 3D printer, it’s easy to form ShapeLock into suitable filament or rod so you can print parts directly in solid polycaprolactone. You can even add stuff to the molten ShapeLock to change the properties of the finished plastic. For instance, adding carbon black allows you to produce a homemade conductive plastic which, rolled into filaments and fed into an extruder, can be used to print electrically conductive parts.