how about a backyard mechanic fix....that has worked for several years on mine.
If you cannot tighten it up the correct way, do what I did. I packed the little area around the ball with nothing more than a peice of a paper towel. Screwed it back up there and man is it nice. No more flopping around and can still be adjusted nicely when your passenger somehow always bonks there head into it. I have another decent mirror for mine, but havent had to use it...I think I have a better set up now than when new. :)
just a thought.
-RPM