The water pillow (Remember waterbeds?)

The Ancient Egyptians had another solution: a carved wooden headrest which fit behind the neck to hold the head in alignment with the backbone. You can try an experiment yourself, by pulling a sock over a suitably-sized bottle, and placing it behind your neck. The bottle needs to be the right size to fill the gap without lifting your head enough to kink your neck. If you can find the right combination, it's surprisingly comfortable. You should try it.

If you're thinking 'use the hot water bottle' I have bad news - it's too wide. You need something narrower, just enough to fill the gap without affecting the other surrounding parts. The object is to have everything in alignment, as the article says above.