ORIGINAL: littlecrankshaf
Try incriminatingly (careful) moving the CG reward. Also, maybe increase flying speed during knife edge. The goal is to lessen the amount of rudder used to maintain knife edge flight. The rudder, by design shape, will impart a roll tendency.
Less rudder application needed...a lesser tendency to roll will be imparted.
Good luck
I think a better solution would be to move the CG forward and add some positive incidence to the wing. Moving the CG aft will just make the aircraft more unstable.