Flap has quite a large gap dimensions, so that cold air can flow anyway

The flap has such great production tolerances and thus gaps that the cold air can flow anyway. While it is significantly less cold air but seal does this flap. It would be desirable if the lid had significantly smaller gaps and thus would be a lot denser.
Comparable non-return valve has a rubber membrane on which the flap to close to 100% a tight seal.