Butyl Tape vs Dicor Self Leveling

put butyl puddy under anything, than tighten down screws a couple inches apart, and the mounting flange will be bent down where screws are and bent up between screws where butyl did not squeeze out. not something I would want to do with plastic or any metal flanges.

I would glue it down with dicor, than seal the screw heads and sides of flange with dicor. But that is just my opinion.