My masks aren't brilliant, but they're good enough that people have complimented them. We like the pleated ones more than the shaped ones.
I understand elastic braid and quarter-inch elastic are not to be found. If you don't have a stash, I found a work-around: a clean shoelace, the kind you pull from your athletic shoes before you throw them out.
Mr. Maryn's mask has a shoelace that he prefers over the elastic. The bottom of the mask has a loop of lace you put behind your neck. You pull the mask into place and pull the ends of the shoelace, keeping the length of the two ends about the same, until the loop tightens to a comfortable level. Put the two free ends behind your head, above your ears or higher, and tie a bow. If necessary, adjust the mask's gathers on the side. Easy-peasy.
If that doesn't make sense, I could be persuaded to do pictures.