Wouldn't worry about the top of the flue... the heat you'll get from the burner and the flue will be enough as long as you're not just open airing it.
You won't choke it with a coil inside... besides, you can always add an air intake to one side... slip of metal plating, rivets to the main burner, and two holes drilled for air entry. To close off the air, turn the metal plate... job done
You won't choke it with a coil inside... besides, you can always add an air intake to one side... slip of metal plating, rivets to the main burner, and two holes drilled for air entry. To close off the air, turn the metal plate... job done