What is used to attach the louver and pan to the hood ?
Correct. Similar to this but with the rubber gasket as shown in your picture.Usually a pal nut. Something like this I'd guess, or maybe with an open top if the studs are really long.
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That’s what I used when I couldn’t find enough of the bell nuts. Finally saved enough lunch money to afford the correct hardware.So basically a nut and washer with a firm rubber washer too