As long as we are talking rooftop A/C, I found great A/C filters on Ebay. They are 3 piece mesh, fiberglass & charcoal filter. You trim them yourself and they are much better than the foam filter. I did not have room for the charcoal filter and did not use that part. They would be good for people with allergies. Did not have any problem with air flow.
Just look for RV A/C filters.
Mike