The I coated the WonderFlex jawbone with spray glue top and bottom, then lightly sprayed one side of a piece of batting and wrapped it around the outside of the jaw.
Then I re-sprayed the inside surface and stretchd a piece of the blue cosmic lizard spandex around that; it wasn't sprayed, and so it only stuck where is was stretched around and pressed into the inside.
Finally, I sprayed the wrong side of a carefully shaped piece of the black cosmic lizard and pressed it down into the lower palate, sealing all the edges in place. Two stitches with carpet thread hold the teeth in place, and two pieces of cord tie the jaw to the hardhat inside the head. Two loops of carpet thread limit how far down it can drop until I can find the springs I bought to do the same job only bouncier.