To finish off the two eye openings on the mask, as well as the respirator openings I am stitching on leather piping. The mask parts are 4-5 oz leather, while the piping is only 2-3 oz, so it will more easily fold.
The pattern for the piping is a rectangle. Here you can see the piping piece at the bottom, and on one of the eye openings the piping has been attached. In addition to looking better, it makes attaching the resin eyepieces easier.