I'm using the pattern supplied by the hospital asking for masks and my Mom is reading into a better filtering fabric for the interior as well.
My mother is the master seamstress, so I'm going to wait for her to tell me what is best for an interior filter but I'm using 1800 thread count tightly woven cotten sheets for the interior fabric. It will be softer and safer according to what I watched...who knows, maybe this is just to make us feel better but I feel like anything is better than nothing!