A two part filter design intended to increase the filter surface area. More effective filtering media has greater resistance to air flow and by increasing the filter area it easier to breath.
April 9 2020: added a reusable threaded cage to hold the filter edges closed. Scale the bolt to create a little more nut clearance like 0.9975
The adapter thread is M32x1.5
DIY solution if you can't get standard 3M filter cartridges. This adapter allows me to attach other filter media to my 3M mask mounting connector. Two round disc of filter media are required. Cut a 32mm hole in one and pinch seal it to the adapter with the threaded spacer. Place a second disc over the threaded spacer and seal to the first with glue around the edge, a cable tie, some tape or....
For the filter media you will need to be creative, possibly a paper coffee filter? NIH DIY fabric mask https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3373043/