I think it's a tricky line between helping to keep room air from escaping, and hindering exhaust flow at startup and possibly running. The prevention of downdraft is obviously a good thing no matter what.
In any event, I wouldn't use one that had plastic involved, but stick to all metal, just in case.
That's my roommate. He's kinda weird, but he always pays his half of the rent. And he gave me this tiny ad:
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