A 6.5″ X 12.5″ MLD 20D flat stuff sack (0.4 oz) already being used to pack other items and a foam sit pad (1.0 oz) measuring 11″ X 15.5″ unfolded (11″ X 4″ X 1.4″ folded). Sit pad can also be utilized with frameless pack for structure and cushion against back (bottom pic), so triple use really.
Flat stuff sack must be just the right size though or the soft items you add to the folded pad will spread out and not maintain the height you need for a functional pillow. Too small and there’s not enough room to stuff the extra items inside.
Along with the folded foam pad simply add spare socks, stuff sacks, toilet paper, beanie cap or a host of other soft items that you’re already carrying anyway and you’ll have a 3 1/2″ thick by 11″ X 5″ pillow….and it will weigh absolutely nothing because the stuff sack and foam pad are dual use (or triple use) items. I’ve slept on it a couple of nights and not quite as comfortable as a designated pillow but for zero weight it’s just fine.



