After going through the disassembly to reach the evaporator, there is probably not an easy or air-tight way of installing a filter that can pass both outside and recirculcted air and still be less than a PITA to replace. Not very practical.
I was surprised to see the amount of dust that had collected on the upstream side of the evaporator fins after a quarter of a million miles. No wonder the air flow had been reduced. This dust layer was probably performing the function of a filter as it continued to grow. A terry cloth towel and vacuum cleaner were needed to remove this filter. Noticeably more airflow.