An original can be sculpted out of wax. Or the original can be fashioned by cutting up sections of other products, gluing them together, and filling seams with clay or wax. The molded prototype will be a one-piece solid. These materials can be selected to affect the hardness and flexibility of your prototype. Use rubber bands instead of springs wherever possible.
Springs are more difficult to adjust.
Many times you can make elements of your invention prototype from paper or cardboard. Pins through the pieces can act as axles or pivot points to see how parts will move together. String can be fastened between pieces to act as linkages. Thicker sheets may need to be cut with a hobby saw. Also, cyanoacrylate and hot-melt glue.