We took the montana mask, found at makethemasks and changed the size to be smaller and the opening to be circular. This was for the purpose of N-99 filters that are 2" and are used by allergist in a spirometer. We have also used the Rigid VF6000 filter vacs as well as the Filtrete 1500 and 1900 filters and cut into small squares. On the inside we have lined them with rubber window weather stripping for a tighter seal. What makes our unique is that if you have an allergist in town and can get access to the N-99 circular filters used by many of them, that our design was made for, then this is the best way to go. I have included a picture of the filter a well as the packaging.