Not everyone has a plaster face cast to use as a base for mask-making projects. Or sometimes you want to build a mask with a different shape from a human face. If you are willing to do some sculpting, here's a solution to that problem.
First, find a clean 1 gallon nursery pot. If you have a few to chose between, you want the one with the thickest, stiffest walls.
Next, you want to cut the nursery pot in half:
If the open end (what was previously the top of the pot) seems floppy you can use some masking tape to help stop it spreading out, as seen in the top-left of corner of the above photo. Or you could place one half of the pot right on top of the other half to make a stiffer base for your sculpt.
Now its time to sculpt a face! (see video clip above). You can purchase sculptor's oil clay from the art store, or look for Plasticine clay in the children's section of most craft stores. (I haven't tried it yet, but I suspect that re-used cheese wax would work great for this too!). You don't have to make the design very detailed, you just need enough of a shape to build a mask over. A form like this works great for the burlap bioplastic mask-making method, and could be adapted for paper mache masks as well.
Helpful tip: Use a white crayon or oil pencil to draw facial features on the pot before you start sculpting. If you plan on making a wearable mask, measure the distance between your eyes, and the distance between your eyes and your mouth, etc. and use those measurements as a guide when you draw the features.