I have modified the Montana Mask to make it easier to print, a little more robust, and the filter insert fits better and is more of a one size fits all.
Adjustable knot if you are using shoe laces or non stretchy fabric for the straps. HERE
Filter materials: Surgical mask is best, hepa filter material like vacuum cleaner bag is good, 4 layers of T-shirt fabric is ok.
I have added 1/4 inch thick foam to the inside rim of the mask to make a better fit for smaller faces and it makes a great seal. Every hardware store will have weather striping foam tape.