Sub fins and finlets ...The finlets on the stab tips help , but also try a big sub-fin under the tail. Same as what Jim mentioned...With all that float area ahead of the C/G, the sub-fin might help..And it won't do any harm... Plus also differential on the aillerons...
I had a 72" WS beaver that did the same thing..My only fix in the end was to mix aileron and rudder...