In the interest of progress, I could easily use the power of CAD to draw any determined shape and post the file for the wing tip. But, I would like to do only for proven gain.
This is fairly simple statement. i.e. With nothing on the wing, it stalls/lands at say 34 mph With- 32 mph and stable Easy math shows that to be 6.25% improvement. Fairly large.
That being said, if all the work that anyone does, cannot be measured- i.e. NO gain or loss Then it would just be a exercise in creating a "look" and adding some weight to your plane.
Please don't read this as if I am thinking this is a "waste of time/material" I think it very well could be a good addition to your aircraft and it would be nice to be building it, (the wing tip) that would be of gain to your flying.
I know drawing this shape could take many hours at the table figuring out just exactly what shape you want. Thus my offer, if someone comes up with a proven winner, I will draw it out to your specs. And post. The power of CAD lets me do most anything. With the simple-list being, wing rib shape, plus, 1" or 2" or what ever, the entire circumference of the wing rib shape. Again if this shape is not proven to be of gain, I simply don't see the need for the work.
Just offering my service on this for the gain of all.