Been thinking about it...you could make one out of some climbing rope, and a carabiner. If you need help with the prussic knot go to http://www.seakayak.ws/kayak/kayak.nsf/NavigationList/NT000B1A26and scroll down to the knot. You should be able to get away with 8ft of rope.
I made one for my brother and he loves it. Ensure you get a carabiner thats rated for climbing...not one of those "gear" holders that are 2.99.
A good carabiner will run you a few more bucks.