Here’s what I did with the fold over elastic .
I made the masks that have the seam down the center and used the full width of the elastic for the back of the head. I have a large head and so the top elastic measures 11 in un-stretched and the bottom 9 in un-stretched. It fits snugly but not too tight nor too loose, and slips nicely over the head with the top band going up over the ears, and the bottom sitting at the below the head and at back of the neck.