Product review (3 stars): Works great, but weak instructions
When I first got this, I struggled for about 45 minutes to figure out how it worked.  Like one of the one-star raters I couldn't figure out how to make it fit to the old 3-bolt flange without falling through.

The problem is the instructions:  they are very detailed about fitting a rubber adapter to one end of the new hard plastic piece.  But then they tell you to take the other rubber adapter  and put it between the 3-bolt flange and the new plastic piece.  What they DON't tell you is that you have to fit this other adapter over a small rib on the hard plastic piece - just like you had to do at the other end.

A simple inspection of the pieces does not make this obvious.  But once you do it, the now-expanded rubber piece fits very snugly against the old 3-bolt flange.

It would also help if their instructions didn't look like they'd been printed with a 30-year-old copier that was low on toner.