I use a lot of masks for my props because of storage issues as well. So when I do, I use a gallon milk jug under it. This fits most adult size masks. And you can stick a glow stick or some form of lighting in it and get a glow thru' the eye holes (tape any other holes over on the inside of the mask, like for ears and nose).
For smaller masks I use juice bottles like half gallon apple juice. I stuff a grocery bag or two in the top if the mask wants to sink onto the flat bottom of the bottle.
If I have a partial mask, like just face w/out hood or hair attached, I put it under a hooded shirt, fashion a hood out of scrap material, or make yarn hair pieces that I pin on.
The best part is I can store all my masks in one bin and throw away the bottles and jugs!