Try to find in your LFS or Internet the filter I pictured on this posting:
It is the replacement sponges for a nano filter. I can't remember the brand right now. Those are 4 squares inside the plastic frame. Ask your sellers of something like that.
That stuff looks perfect. It will catch large chunks but let micro debris through.
Honestly, I think it's just AC air handler filter material. This can probably be purchased at home depot and cut into appropriate sizes.