I had the same problem with my aftermarket knob (not a MOMO, although I am getting one of those soon).
If you don't want to spend any extra money, you can do what I did. Pull the boot down some so the metal shaft is exposed. Take a strip of cloth or paper towel and wrap it around the shaft, then secure it with a twist tie. Wrap that up in electrical tape.
What you have done here is just enlarged the width of the shaft so the stock boot ring has something to fit around. Then just pull the boot ring over the taped paper towel and it should stay put (instead of rattling around). It won't look perfect but it's a free solution.