Gizmo3D, in the 70's I had an Ugly Stick with wingtip damage so I chopped them off flat and tried various screw on plates. A very large plate about 2" larger than than the airfoil appeared to allow slower flight and roll rate seemed to decrease.

I cut one side of the plates off and tried with the extension above and below the airfoil. Having the extension hanging below did not seem to make any difference. Having it above seemed to have the same affect as all the way around. I had a lot of advice from the field experts in my experiments.

Covering the top of the wide aileron gap with a strip of Monokote made a bigger difference.

Overall I've found that it's difficult to tell the difference of some changes. Tacking a square piece of balsa 1/2 the span and about 1/4 cordwise back also seemed to allow slower flight.

Perhaps when INEXPENSIVE in-flight measuring equipment becomes available more will be learned.

I had over 200 flights on the plane with a lot of experimenting, read a lot of theory, and to this day I am still not sure of all my results.

Rich