Glue them, pull in tight with some decent tape. Then seal the whole skull with a 50/50 mix of PVA glue and water.
In the future, don't boil them too hard out. Simmer them until the flesh/membrane on the face starts peeling, them water blast. In my experience you are better to try and water blast earlier rather than later, as it avoids the bone becoming brittle or going chalky.
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