there is a type of wood which I think is called aircraft plywood (used to make hobby airplanes)which JHC uses on stiff puppet mouths. I've seen it used on elmo, it's like a really light and thin plywood. (Perhaps it is balsa?) Most of the big muppets don't have mouthplates (not a hard one anyway), I'l try to explain how they work. First there's the foam rubber shape of the upper mouth. This is either carved so that you can put your hand between it and the mouth plate or if it is extreamly big it's carved hollow and a sheet of foam is glued on to hold your hand in place. Then theres a foam sheet where the upper mouth plate should be. In the lower mouth foam is also used as mouthplate. However, to keep the thumb from just bending the foam up, the thumb cup (heh, funny word)is actually fastened to the foam under the mouthplate instead of on the mouthplate itself. The thumb cup is made by folding a thin peice of foam down to shape an upside down U. The two edges are then glued on the lower foam shape, and might be reinforced with more foam sheets.