Here you can find a range of breathing mask 3D printable models: https://top3dshop.com/3d-model-finder/?q=breathing%20mask%20...
This video includes a good tutorial on how to 3D print, assembly and finish a working breathing mask. I strongly suggest watching it: https://www.youtube.com/watch?v=77PgC3kMrFs&feature=youtu.be
https://twitter.com/annimaniac/status/1244124967521751041?s=...