To get rid of the coupling, you need wing anhedral. In other words, the wings will have to sag towards the bottom of the plane. Another option is to tilt the outer cabane struts inward at their tops.

I cannot tell you how many degrees it will take of each to totally eliminate the coupling. It would have to be a trial an error affair.

What you were planning to do with the stab will affect the pitch coupling, but would not really change the roll coupling.

George Hicks has a forum, and is very knowledgable. He does this stuff for a living. If anyone can give you an idea of how many degrees anhedral to start with, he would be the one..........

CJ