I tried rather large winglets, actually more like tip plates on a competition fun-fly model. I had to give them nearly as much area below the wing as above, to reduce their dihedral effect. Made them of epoxy laminated balsa, with hardwood hard points for mounting by small nylon screws so that they would break away without damage on ground contact.

The reason for my tip plates was to reduce induced drag. A slightly longer wingspan would reduce induced drag much more efficiently, without nearly as much increase in parasite drag, but a longer wing would also reduce the roll rate. With the tip plates in place, the model loops considerable tighter and faster, and also handles nicer, with very friendly behaviour much deeper into stall, due to its greatly increased yaw resistance. As you would expect, the bird also knife edges far better.

Also tried ugly detachable vertical airfoils at about half span on wing of a piped Webra 120 powered, frantic flying 9 pound aerobatic monoplane with coupled flaperons, trying to make it knife edge as well my bipes, with mixed success.