Had a little fun with foam filter media.
I had some UNI 60ppi (fine) stuff sitting around so I tried it out:
Also got some UNI 30ppi (coarse), which is obviously too coarse to be much more than a big ol' chunk filter
The two foams stacked up vs. the Merv4 media
I just focused on the airflow since they all had about the same sound
The fine foam worked about the same as the Merv4 stuff. The coarse foam actually provided noticeable more airflow but it not a very good filter. This could be a good compromise if someone doesn't really want dust/pollen filtration but just to keep out leaves and stuff.
I'm all out of ideas for now!