Didn't your FF kit come with strips of anti rattle tape? If not, contact Michael. I believe that's normally included as part of the speaker kit, at least it was on my 10" kit.
Do just one strip on each side and see if that's enough. If you put too much tape the glass gets hard to move. I think on mine I have 1 1/2 layers (2 on one side, 1 on the other) and it's pretty tight. Good part is the glass won't slide out when you remove the lock bar.