Whole House Fan
Whole house fan lids must be treated with a whole house fan box, made from 2” rigid foam board.
It should have a removable lid with a friction fit piece underneath the lid that fits into the side walls. If a whole hose fan box cannot be built due to clearance, a batticdoor.com "whole house fan cover" should be offered to stop any air leakage from below. These are secured from below the opening.