NBC had an articl on fabrics to make masks : https://www.nbcnews.com/health/health-news/making-your-own-face-mask-some-fabrics-work-better-others-n1175966
"The best masks were constructed of two layers of heavyweight "quilters cotton" with a thread count of at least 180, and had thicker and tighter weave.
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Lesser quality fabrics also worked well, as long as they had an internal layer of flannel.
"You do want to use a woven fabric, like batik," Segal said, "but you don't want to use a knit fabric, because the holes between the knit stitches are bigger."
In other words, if the fabric allows for a substantial amount of light to shine through, it's probably going to allow tiny viral particles through, as well."
I'm holding up my wool felt, and it's hard to see light through. Maybe wool sandwiched between two layers of quilters cotton would be good?
in New Zealand says that "Our mask’s distinctive shape and unique wool filter reduces the build-up of moisture and heat which is common with other masks. The filters have extended particle capture ability, resulting in superior performance."