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.
"To do good, you actually have to do something." -- Yvon Chouinard
Quality Agriculture, The best book on regenerative agriculture wisdom
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