This sling is flexible, comfortable and does a good job of immobilizing the limb for sprains and simple fractures, but it is not rigid. For worse injuries, the bark splint is a smarter option.
Bark Or Stick Splint **
This first aid trick is obvious and fairly easy. Wrap the limb in any bandages or padding that you have available. Next, find some bark strips or sticks, and tie them around the injured limb. Wrap the line around the back of your neck like the sling already mentioned, and tie the free end of the rope securely. Make sure that you don't tie the splint, sling or any of the wrappings too tight -- this is injury support, not a tourniquet.