Honda's quick fix for the car's cooling

In fact, it's a quick fix for heating the car, as it reduces cooling capacity. Modern F1 cars use airflow in the sidepods to pass through the radiators in order to cool the engine. Behind the heat exchangers, most teams have shark gills in combination with a (closed) chimney to quickly allow the hot air to escape the sidepod.
While such gills are a perfect compromise between cooling capacity and drag, it is always better to have less openings if that is possible. So, Honda have quite simply blocked the upper gill. This will prevent hot air from intervening with the airflow over the sidepod. On the other hand, one can assume that a team like McLaren would do a little more effort and make sure the surface is smooth, rather than still be the cause of some unnecessary drag.