I've been making similar masks-2 layers of fabric with a layer from a vacuum cleaner bag in between. But I have had success gluing the seams with white glue (the non-toxic elmers stuff) instead of sewing. A good option for those of you without a sewing machine.
PVA glue is water soluble, so if you wash it you'll need to rebuild it. For something like this you don't need a sewing machine, just need a needle and thread is enough.
PVA glue is not water soluble after it has cured. It's a common myth which is entirely false and easily testable.