Here's a technique for making low-cost, light-duty below-the-elbow prostheses from 2-litre soda bottles from Instructables user CIRnetwork:
Plastic Soda Bottle Prosthesis (via Make)
In resource-limited areas worldwide, individuals with amputations may not be able to gain access to prosthetics devices due to a lack of the materials needed to fabricate them. This simple technique utilizes a 2-liter soda bottle to create below-the-elbow prosthesis suitable for a number of 'light duty' activities.
Please note well that this technique is intended for use by trained prosthetists and it is intended to be preformed only using a plaster cast of the residual limb.