I actually made a work at school a few years ago wind tunnel testing wingtips. The measurements weren't all excact but i received some ok data I think...

I used a test model with a span of 20 cm and a chord of 16 cm, airfoil Goettingen 604. One problem was of course the size of my wind tunnel and the interference with the walls but they were all measured the same way...

The tips I used were: straight cut, rounded, pointed i.e. triangular surface attached orthogonally to the straight cut one and then blended to the wing with solar film (ugly stik), winglet (rounded edges) and endplate (race cars).

My measurements were made using pressure differences and, again, can of course not be compared to the big guy's tests...