Well, if that's the size you want, then perhaps you could do this:
- cut the mandibles off at the circular edge along the ear pods and the edge between the indent and the flat top surface of the mandible (just the two cuts and it should be fully separated if you know what I am getting at)
- Cut a slot in the forward arc of the circular ear pods
- Extend a flap rearward along the inside surface of the mandible to fit in the ear pod slot
- Extend a flap downward from the cut, indented edge (seen above the mandible piece) that will be a layer behind the mandible when in place.
That way, you can slot in the mandible at the ear pod, and velcro or double-stick-tape the inner surface of the mandible to the new sub-layer coming down behind it (whichever will hold for filming) - with a tight enough tolerance on the slot, you wouldn't have a gap at the ear pod and it would look the same. at the top surface of the mandible, if that's got a gap, maybe use an adhesive or something that can be just cut away with a razor for removal between scenes. Not perfect, but might work.
Alternately cast it in a very firm rubber so it'll flex for the actor but retains its shape easily and durably.