These masks are made from 100% cotton, aka quilting cotton, and elastic. The tighter the weave, the better. If you're unsure, give it the flame test: put two layers of it up to your mouth in a mask-like fashion and try to blow out a candle. If you can't, that fabric is a keeper.
Your elastic length needs will vary, but here's a rough guide: I use 8" (20.32cm) pieces for my husband's masks (he has a beard) and 6 3/4" (17.145cm) pieces for my masks.
These masks are pretty cheap to make, especially once you get a lifetime supply of elastic from the Bezos Barn. If you already sew, it's likely that you have scraps that are big enough to make masks -- you need two pieces that are 6" by 9" (15.24cm x 22.86cm). I made a handy pattern that you can download below.
***Metric pattern coming soon (15.5cm x 23cm)***