The difficulty is that the a/c unit blows quite hard, so if you dont get a reasonable seal than
- Hot air leaks into the room
- Soot is loosened and blown into the room. Not much, but enough to be a nuisance.
To get a seal, the baffle has a sliding component which pushes up into the fire hood, and a push-fit secondary baffle at the top which wedges against the hood to form a seal. Draft proofing around the inside of the baffle makes a tight joint.
As the hardboard gets fairly warm, we avoid heat reentering the room by radiation by using a polystyrene and foil liner is glued to the inside of the baffle.