I have been working with winglet design for some time now. I have used the stock ones, 2mm Coroplast and .016 carbon fiber ones... all in various sizes and shapes.

Conclusion: Coroplast it seems is the material of choice. It weighs 1/3 less than .016 carbon fiber and 1/2 less than the stock plastic ones and is much stiffer and cheaper. (The CF was $38 a sq. foot!)

I have made a design that hangs 1" below the bottom of the wing, this seems to make a noticable difference in the hard turns, like in combat or aerobatic manuvers. I have also double slotted the trailing edge area of the winglet to accept a single piece of 3/4" fiber strapping tape - strong and easy to attach. One piece on the nose and you are done.

I have finalized what I consider my best design as of April of 2002 and if you look at these photos you will be able to make your own performance enhancing winglets!