Speaker smaller than hole by a few milimeters. What to do?

I have 4" speakers on a Toyota. Toyota "created" a new speakers size called T4, which are basically 4" speakers with a weird screw configuration. I managed to install the speakers, but it isnt completely sealed. There is some space between the speaker and the frame holding it. What can I use to... show more I have 4" speakers on a Toyota. Toyota "created" a new speakers size called T4, which are basically 4" speakers with a weird screw configuration. I managed to install the speakers, but it isnt completely sealed. There is some space between the speaker and the frame holding it. What can I use to fill this space up?
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