Changing from retainer to stiffener

Mid 2021 update: High thread count cotton and/or multi-layer polyester satin isn't a good choice for high filtration masks. They're not breathable enough at mid filtration performance. And additional layering to get to hight filtration still only decreases breathability further. With that they get hot and uncomfortable as well as giving your chest muscles a good workout.

So I’ve 56 masks in progress (filter is 2 satin layers, and a 180-thread cotton outer): 

Two short plastic strips to insert between the cotton layer and the 2x satin layers. Sew line marked

Curved line sewn through 6 layers of fabric, missing the newly inserted plastic strips:

A cut outside that sew, and then some satin-acetate bias tape added.:

Also a wee 'center' line marked (both sides):

The two bits of plastic "shuggled" into a place where they are applying max pressure to the center line, and sewn with a button-hole stitch. My needle can pierce the plastic just fine, but it would be possible for you to line it up to just miss both sides:

Insert a zip tie pushing as far as you can. 

Hopefully it jams and doesn't emerge from the other 

end of the bias tape tunnel:

Pull it out a few mm, cut it off, push it back in:

And sew that end shut with the button hole stitch:

The change from the visible retainer to two invisible stiffeners means the masks will be more pleasing to the people that own/use them. Or less unpopular, as you're still breathing through three layers of fabric. All in the name of "breathability"