Can you go any smaller than 6mm? A mouse can fit through a 6 or 7mm hole, so that will stop rats but not mice.
i was going to double up the layers which should make it much less possible and then I may hit it with a smear of expanding foam just to close it up.
@HatBeard started
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Can you go any smaller than 6mm? A mouse can fit through a 6 or 7mm hole, so that will stop rats but not mice.