Nothing special, whatever's available. It just has to support my head at the right height, as set by my shoulder size when sleeping on my side. If it's too sm

Nothing special, whatever's available. It just has to support my head at the right height, as set by my shoulder size when sleeping on my side. If it's too small or squishy then my head will droop too low and cause neck strain. When camping I'll just use my backpack with a hoody thrown across it to soften the zippers etc. All that matters to me is the resting head height... I don't think the composition is important.