To cure the 'floppy' nature of these masks, once you have cut it to the final shape,
Turn it over and paint on a thin layer of car repair resin. Allow 10 mins for it to cure then add a second layer adding a layer of tissue to the resin. You will be amazed at how strong the masks becomes after it has cured. Leave it for a couple of hours and then trim up with sharp scissors.
Sand any sharp or rough edges with some normal sandpaper or use your Dremel which speeds it all up a bit. The tissue will block off the poor nose and mouth holes.