Updates:
2020-05-03: fix floating strap mount, thanks to jpsaila for pointing this out
2020-04-25: flex friendly version https://www.thingiverse.com/thing:4316736 :
2020-04-18 : minor updates, added narrow versions, step files
2020-04-06 : new cap and cover for thicker pads
2020-04-05 : new grill and strap mount design
2020-03-25 : uploaded new version of 2 point strap mount, also in 4 sizes: the original (100%), 105%, 110% and 115% scales, all using the same filter cover
2020-03-25 : uploaded version with 4 point strap mount, in 4 sizes: the original (100%), 105%, 110% and 115% scales, all using the same filter cover
2020-03-21 : small improvement on the strap mount, STEP files uploaded for those who want to remix, bigger version 5% bigger than the original

2020-03-19 : based on Euter's comment, sealed up the mounting and strap holes. Redesigned strap mount.

Overview:
This face mask was originally designed for circular cotton pad as filter. Cotton pad is roughly 58mm in diameter and are readily available in the cosmetic's section of most stores. Pads can be made from HEPA vacuum cleaner bags, also several can be made from N95 masks, and other filtration media where applicable. Mask available in 5 sizes

Printing:
1) Choose mask size (may need experimentation which one fits you best)
2) If using only cotton pads print cotton filter
3) If using thicker pads or you want to stack filters (e.g. hepa and activated carbon filters) print filter cap and scaleable cover, scale cover along the z only, should be roughly 7mm minus the total thickness of your pads and interface covers, provide enough room so the filters are squished just enough to seal the sides.
4) If you want to use the filter cap and the scaleable cover with cotton pads, scale the cover to 6.2mm on the z only

Usage:
Sand rough edges that will be in contact with skin. When mounting the filter to the mask, rotate counter-clockwise until you hear a click. Release filter by rotating clockwise.

Replace cotton filter as often as possible and wash mask with warm water and soap.

To improve fit, seal and comfort, I tried Nexcare waterproof tape around the rim of the mask, P-type weather stripping applied inside the rim works well as well, Neoprene foam tape end etc

https://www.homedepot.ca/product/m-d-building-products-3-8-inch-x-3-8-inch-x-17-ft-small-premium-rubber-gap-seal-p-profile-white/1001121260
https://www.nexcare.com/3M/en_US/nexcare/products/catalog/~/Nexcare-Absolute-Waterproof-Tape/?N=4326+3294529207+3294631805&rt=rud
https://www.amazon.ca/gp/product/B01G382DX4/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

For 80mm N95/N99 pads, see this : https://www.thingiverse.com/thing:4235068

Disclaimer:
Use at your own risk