Earlier, Cv wrote:

...Instead of carefully crafted, expensive
winglets, wouldn't straightforward flat end
plates serve nearly as well in reducing
wingtip vortices and improve performance.

Have they ever been tried, and with what
results ?

Dick Schreder tried some fairly simple endplate type
winglets on the HP-18. They didn't work very well.

Winglets must be carefully designed and executed, otherwise
they are more likely to reduce performance than improve
it. The flows in the tip vortex call for carefully
optimized planform and twist distribution.

Based on the winglet design that Steve Smith did for
my HP-24 wing, I think that it is generally possible
to develop winglets that are fairly simple in geometry
and construction, and that are not very expensive in
terms of materials. However, they would still have
to be carefully made, and that's usually where the
money goes.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24