3 Since yours is missing, is the hinge still there? If no hinge, check the vent box itself where the hinge would go. It might be damaged in that area and would need repair.
I will check and see what I have in regards to a door.
EDIT: it seems on most I’ve seen, the top pivot on the hinge gets rusted and then breaks off trying to open the door or the hinge gets pulled right off the box.
