1176 gsm of spunbond NWPP in a washable mask
Start with the 16 of this blog entry, and create an insert of 12 layers:
Verdict: still breathable, but much less so than the 16 layer one on its own
The insert's pink template was made freehand.
As a consequence it is a little short in the throat area,
which you can see in the last photo.
For reference, the mask I just made is more breathable than
an FFP3 mask I've had for a couple of years: