I have done a little work with "tip plates" actually called endplates. It's my understanding from reading and experience that in order to be effective they need to around 10% of the wing area. This is quite large. In full sized aircraft and racing cars they are used to increase lift, not reduce drag. Generally you can increase lift by up to 7% with a drag increase of 1% to 2%.

I used them on a combat flying wing I used to market, they looked cool and supplied some scuff protection of the tips on landing. Also by toeing them in about 1/2 to 1 degree it tends to stabilize the wing in flight.

IMHO you'd be better off studying Hoerner Tips.

Stan D.