When I go camping, I use a standard 1L Nalgene bottle as a pillow. Sometimes I'll put it inside the sleeve of a fleece but it's actually fine on its own. Maybe it's just because I'm usually tired, but I find it fine to sleep that way. Then again, I think a lot of people historically would have been pretty tired at the end of the day so maybe it was easier to fall asleep then than it is now.