Threads kinda old, but if you're still looking for suggestions...
I used foam for my latest mask (the kind they use in walls, to keep the pink insulation from falling out a.k.a. smooth rigid white EPS foam) bought at my local hardware store, a started with a big sheet between 2-3 inches thick.
Cut out a rectangle slightly bigger than the size of my mask, then saw it roughly into shape with a sharp knife (DANGEROUS - use common sense!). **Note - this part can get messy, since the pieces of foam stick to anything, everything with a static charge. Better to do this step outside
, or someplace with a workshop vacuum.
My mask was for visual purposes so didn't require any eye holes or facial modifications, but you could carefully dig out space for your face & cut eye-holes in the foam. I've heard Cheese-cloth is good for see-through coverage, though you may have to dye/permanent-marker it black.
After it's reached a good basic shape, it's time to pull out the sandpaper! Sand it down, starting with a lower grit (the higher the number the smaller the sand-particles, and the smoother your surface will end up) and work your way up to higher ones - I used 80, 100, 120, and 200 grit. I also used my hands for most of it, though a sandpaper-holder would've been nice. With time, effort, and patience, it will end up very smooth to the touch.
On to the painting phase - I used white gesso paint for the base, then added mixtures of black and white acrylic paint (cheapo, from the craft store) for the eyes, mouth, and cheek markings. This part was a bit nerve-wracking (my Crayola brushes didn't give me much precision) and I had to touch up spots over and over until I was satisfied, but it worked. If you want it easier, I'd try cutting out the shapes you want in paper, attaching that to masking/painters tape, then carefully cutting out the paper part, leaving an eye/mouth-shaped stencil you can lay over your mask and paint over, then remove after the paint has dried. Didn't try it myself, but I wish I had.
To prevent the foam from uncomfortably rubbing your face, hot-glue some dense foam (like the kind they use for furniture) onto the inside of the mask, about where your cheeks will be. Also, if you're strapping this to your head, I suggest hot-gluing a cardboard-rim around the inside, and attaching any stretchy-strings to that
, since foam will crumble from stress concentrated on a small area.
Well, lemme know how it goes - here's a picture of my finished mask!