My best results have been with the same tip as in hattend's pics...

The way I make them:

Tack-glue a thick sheet balsa tip plate, then triangle stock to the wingtip such that the top of the tringle stock is a 45 deg corner and it as close as possible parallels the aft 2/3 of the top of the rib.

End view... carve the top to match the upper surface of the wing. the lower part, do the same for the front appx 30%.. and from there back its a straight line to the bottom rear corner of the triangle stock.

Front view you carve to a pleasing rounded outer shape. The inside is holowed out and sanded with a dowel wrapped in sandpaper.

Pull the tips off and hollow them out to reduce weight. then reattach.

Results have been amazing to people that take one look and say it can't work... (many say it can't have much effect if any... but its HUGE!)

I stuck this type tip on a Sig Kobra (.25 size 1970's Pattern design) Max speed was noticeably improved and the plane suddenly was able to float with planes having much lower wing loading (same wing area) and flat bottom airfoils.

I put these tips on a 2 CH slope racer and was suddenly outrunning the kit designer's model... Same weight... no other mods. (and he was a MUCH better pilot...) And the little thing was able to thermal with a Gentle Lady in light lift...