A gentleman at our field has a PT-19 that would tip stall at slower speeds. He solved it by installing what he called a "stall strip". This was nothing more than a piece of 1/2 inch triangle stock glued to the leading edge of the non-dropping wing in towards the fuselage. He said that some full scale planes use the same technique and what it does is reduce the lift of the "non-stalling" wing and essentially evens out the incidence. The piece of tri stock was about four inches long or so, and once covered with matching material worked perfectly.

It is almost too simple, but the PT-19 went from a violent tip stall to just mushing over at the stall speed.

There might be some more info about this somewhere in these pages.