Here’s another source of information on filter material and face mask design. It appears some brands of blue shop towels are a good filter medium. Face mask design pattern is supposed to be released next week.
Business Insider article
Here’s the link to the Suay Sew Shop design pattern and instructions.
Suay Sew Shop Design
A no-sew desing using shop towels (works for other materials too):
No-sew Mask Using Shop Towels