This design is based on measurements from the MSA M13-A2 filters that were included with my mask.
I've added an air channel leading from the external opening to where the air exits the filter pouches. The should prevent the rubber from sealing up around the back side of the opening and creating a breathing restriction.
I wasn't able to figure out how to properly curve a surface in FreeCAD. I simulated by using two ellipsoids to cut the top and bottom of a pad. Needless to say this kludge required a painful amount of trial-and-error to get both the dimensions and positions spot-on. I welcome a remix, or even just a comment with guidance, if anyone here knows how to properly curve surfaces in FreeCAD.
The author marked this model as their own original creation. Imported from Thingiverse.