Soft, thin, "rubber"? turn it inside-out, paint it ,sew on it, stitches, one eye shut, whatever? Put this low-investment on your most energetic actor in a dark corner , and it will be effective, in part, I believe because no one will recognise this "Mask" from any movie, and if they think they do, they will be wrong!
Even tabulating time and materials in such a "smask" (Did I just typo-make that up?) if it gets tromped into the dust, ripped stolen, so what?
The people scared will jump against the "Inventory-taking "customer and accidently knock their tally sheet or mini-computer from their hands as everyone screams and run onward.
So many short-comings ($$) and problems will be overcome via talent, creative actions.