Papier-mache is pretty low tech. Letters sound like they would be of a proper grade of paper to utilize for it, and I'm sure plaster is utilized somewhere within one of the villages or cities. Wouldn't see any issue with that myself. But, alas, I am not an admin nor an avid designer so my opinion means diddly.

To point out, in the Meliashmora there is a mask maker and I believe he makes them out of paper and wood, though I'm not sure. Papier mache would likely be fine, as the glue part of it could be explained away as nearly anything.