Affixing speaker grilles

Is there any special mojo required for affixing the metal speaker grilles that come with aftermarket speakers? The Porsche grilles are actually part of the plastic frames. These like to crack with age. The Porsche dealer wants about $35 ea., just for the plastic frames with grilles. For the same price, one can get nice speakers with grilles. The problem is that the plastic frame always deforms somewhat when screwing down, and the damn metal grilles that are supposed to snug nicely into place, don't. This is particularly troublesome for the door speakers, since gravity doesn't help keep the grilles in place. I tried shaping the grilles to the frames as best I could, but no cigar. What is the proper technique/material here?

Thanks. Brian