It's more likely pitch caused by the rudder than by incidence.
You could work with incidence with shims, with the idea that you don't want a permanent installation.
How to shim is a situation based on how the wing is mounted.
A small amount of shim under the trailing edge, if the wing has dowels at the leading edge and bolts at the trailing edge isn't impossible, but I think essentially it's a situation where the inertial axes of the plane are such... vertical surface relative to the actual rotational axes is the culprit.
I'm betting that hard rudder in level flight results in a roll-pitch down in the direction of the rudder deflection. A couple of my planes can be depended on to do this.