My only suggestion could be to make a splint witn metal/stiff plastic and leather or some strong rope.

place the metal/plastic on either side of the leg, as well as on the "top" and "bottom" and then wrap the leather up and down the two side peices, Useing it to keep the knee stationary.

This will probably require a rather long length of leather. I personally would avoide useing wide strips of leather in favor of thinner and one whole peice.