There's a lot you can do with a bit of wire, a luggage ratchet-strap, and a big rubber band.
In some cases, the best way to repair a guitar headstock doesn't involve clamps and cauls. Depends on the particular damage but this one doesn't need anything more 'specialist'.
Now, I should mention that this isn't always suitable. In many cases, it's a terrible idea. However, with the 'right' break, this method is not only easier but gives more secure crack closure while the glue cures.
A comment made me realise I hadn't actually added an 'after' picture. Rectified now. Finished repair below.