Actually the design is sound, but really needs to be perfected. it doesn't always work because it needs to really fit each wearers individual face. it is the force of the chin and the torque provided by how secure the rest of the underskull is, that determines how well the jaw opens and closes.
the next problem is your mask. Slip latex masks ( especially a nice thick one) wont want to move like this. Gabe made his mandibles dangerously thin. I say dangerously, because when they arrived, the MIGHT have just enough latex to cover the soft foam...but even then, it was a mater of time before they tore or just melted away.
a regular mandible cast, and attached separately ( with its attachment point hidden in a skin fold) could essentially work ..the mandible and the mask would not be attached to each other ...the mandible only attached to a rod the runs to the underskull.
there was a Spanish language version of rudimentary schematics to Gabe's mask floating around here if you can find it ..I doubt it will do much good as I have yet to see anybody try and replicate.