Another stray thought here.
Since the wet cloth works in some situations, I would wonder what a wet towel placed over an air intake for the a/c would do. In the setup I'm looking at here, the "furnace filter" in the winter is also the "a/c filter" in summer, and the rectangular box that holds it all is indoors. When the furnace is running I notice the suction is quite strong and a towel could be taped at the top or held in place by a couple of heavy books. The towel could be wet to start and periodically a person could spray or mist the towel to keep it moist. Not sure if this would help, but suppose the moisture might help trap more stuff.
OTOH, a wet towel might be too restraining and might cause the a/c or furnace to labor too hard. Another thought is that I don't recall the same suction during the summer, so maybe that part is different when a/c is running. I'm certainly no furnace/ a/c expert!
Anyway just another 2 cents...
