close an old hood vent
The 'ducting area' of most hardware stores will carry caps that fit over common sizes of duct. Buy one for the interior and seal it in with silicon as close to the outer wall as possible. On the outside these vents are almost always covered with a downward angled cover. Just spray in "large  crack foam" and you may not even need to move the cover. I wouldn't even bother to conceal the outer cover, houses have these kinds of covers all over.