I would say again to try a spoileron mix. Even with the CG way back some planes wont do it. An example would be a Funtana50X. This is the first plane I got to do an elev. Because of wing rock and other factors I'm sure, the only way it will do one is with spoilerons. Doing them inverted also helps with the wing rock if you have not tried that. Maybe the rudder being in clean air helps. Anyway I posted a few pics of the mix in action.

David