This is for a far away friend... sorry to have been so long in getting these to you.
This is the carbon insert. You can either hand stitch the depth smaller or do it on a sewing machine. It just decreases the depth of the filter. You can trim off the excess from between the serging and the alteration stitching and it will fit better in the shell of the mask.
Above is the shell of the mask and I turned it inside out. Then I stitched by hand to make the depth smaller for smaller faces. I didn't do it on a sewing machine because of the metal nose piece. If you break that metal piece by folding it back and forth, the mask doesn't fit *nearly* as well.
Below is where I took in the mask to make the sides shorter and alter the depth further. I didn't do these tucks (on either side) in the first 2 masks and they would fall down on H's face. Just make a small tuck (3-5mm) on either side of the mask where the metal nose piece ends. This helps fit so much. Then all you have left is to take in the elastic and trim the ends. Hope this helps and, again, sorry to be later than intended in posting.