Attention, still in working progress!!!
I'm working on an idea, a prototype coaxial filter ... my intention is to design the mask too ... the filter cover has the air intakes facing the face to avoid taking direct air from possible harmful particle emitters and to avoid accumulation of contamination ... also the sandwich-filter has a double grid with a protruding part looking at the fabrics and a smooth part looking at the pieces (cover and adapter) ... the filter itself is made up of TNT, make-up remover pad and TNT ... and pretending to add a double coaxial valve to make inhalation independent from exhalation ... in addition, making it coaxial has its reasons: the inhaled air passes through the funnel, reducing the part exposed to the outside and the exhalation goes through the outer perimeter. It has also been taken into account to put a cylindrical filter for exhalation (perimeter yield between internal and external exhalation windows), therefore the double layer on the outer tube in the exhalation zone. So there is more than one plastic barrier in case it leaks for porosity, although pretending to give a whole bath of polyurethane spray.
I checked the airflow and it is more than enough as I see it ... I think the flow is kept within acceptable ranges.
It is also intended to design the mask or adapt it to one that you have already created successfully.
In red color arrow the exhalation and in blue color the inhalation.