I guess you could use the slow messy method in the above link.
What I did was much quicker and easier. I used a small hammer and a sharp punch. I removed the mirror and turned the head to expose the part of the ball that is normally covered. I used the hammer and punch to knurl the ball. I then moved the head some and knurled the other side of the ball. The mirror is nice and tight now and the knurling is hidden when the mirror is returned to its normal position. It is a quick and dirty fix and that works great!