For what it's worth, I believe the Buick Regal's did come with a cabin air filter from the factory. I'm sure someone could correct me if I'm wrong. If you do have a cabin air filter, this is even more important because you do not want the filter getting wet. It could cause mold and actually make the air quality worse than no filter at all.
I labeled this as W-Body because this is a very common problem with the Grand Prix, Regal, Impala, Monte Carlo, etc. They all have the same set-up. Why on earth GM didn't just extend the metal flange on down to sheild the entire opening is beyond me. Seems that would have been much more effective and there's plenty of room between them so it wouldn't have hurt airflow any. Probably just a design oversight.
