The best thing for cree rings I have tried is hairspray on the reflector.
Blends the rings to a uniform smooth spill.
Hold the can a couple of feet away from the reflector so that the spray can dry a little on the way and "dust" the surface when it lands.
There was a thread in the General forum recently on frosting lenses (rather than the dome). Perhaps similar enough to give you some ideas, or an alternative approach.