get 100% silicone aquarium caulk, make sure it is clear.
clean the mask very well with isopropyl alcohol, you may need to use panty hose as a backing fabric (a small patch). You can mix a small amount of oil paint into the caulk to match the color of the mask.
Use the sealer like glue while the mask is on a head form. the head form will help it keep its shape. allow it to cure throughly before removing. Make sure to smooth the silicone over a decent amount of surface area so it will hold well.
For an alternative method (I have had bad luck with GE silicone caulk lately due to a formula change) try this...