Originally Posted by UStik

Flat-bottom wing is a tried and proven concept, especially with RE models. I don't think undercamber would help.

If fuselage shape and canopy really shadow the vertical tail I'd try to get the airflow to it. That's why I mentioned a canalizer but maybe even a row of pins put into the kink on the fuselage's top (just in front of the canopy) could help. At least it would be an easy experiment.

P.S.: As an afterthought, I find the dihedral not overly big. I measured 7° (on each side) and that's less than my 8° rule-of-thumb for shoulder-wingers.