A rear gasket on a front loaded speaker could be for sealing an enclosure that's a closed cab or ported specifically to drive the resonant frequency down as in hifi speaker cabs. I'd use as thin a piece of foam or tape as possible to limit rattles. If the baffle is dead flat as it should be, rattles should not be a problem. Tightening the speaker rim down on a piece of foam could warp the rim and shorten the life of the speaker. This should really be watched when rear loading a speaker as there's a lot of compliance in the cork rim gasket. Bent rim = out of round coil and eventual rubbing and a blown speaker IMO.
When the secrets all are told,
And the petals all unfold.