Remember a few weeks ago when I used alginate to reduce a Jello™ mold? I had some of those smaller forms left over, and decided that one of them would make an excellent chin. It reflects the lines of the top skull (also a Jello™ mold), plus it gives a rounded bottom, which I think is preferable to a sharp bony chin.
I just had to saw it in half, and sand it to fit. I was concerned about the overall height of the skull becoming too long, so I decided not to copy the mandible more closely. Besides the chin, I've drilled small ventilation holes in the eyes, and also created the nasal cavity. It gives the look I'm after, plus provides two more ventilation holes so that the wearer can breathe easily.