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We used a type of vent like this only not adjustable. I can't find a pic of what we used
The open/closedcovers like that are called 'registers' if they deflect air multiple ways they are called 'diffusers' or a combo of the two words...
What you want for a coop would normally be called a 'return air grille' or simply 'grille', they are just fixed vent covers...