Flares which terminate sharply at the mouth typically suffer mouth diffraction more than ones which dont. Just like a speaker box, really. On a tractrix flare, the added flare at the end of the mouth serves to limit diffraction. On other flares which dont have this extra rounded bit, some people put felt or some other damping material around the edges of the mouth to reduce diffraction.

Dont put any absorbing materials inside the throat or near the throat entrance or higher frequencies will be attenuated severly.

I'd add a mounting ring or route out a groove where the edge of the cone/surround meets up with the mounting plate to prevent it from slapping against it at high levels.