I've been putting 12 X 20 X 1 pleated furnace filters between the cover and evaporator coil of my 22, 000 btu window style heat pump for about 15 years now. I discovered early on that this size filter would fit inside the plastic cover and completely cover the evaporator coil, and I have been using them ever since. It serves as my shop air cleaner as well as heating and cooling source. When I'm active it the shop and making lots of fine saw dust, it can fill quite quickly and require changing. I usually blow them out with compressed air a few times and then replace them, since I buy the more expensive pleated ones with the largest filter surface area.
Central North Carolina