If in the back country, think what you can use that is dual use for the things you have on hand already. I still have a sock stuffed with rice , but that stays at home. That was the go to for many years as you can eat it in an emergency.
I have Box to Bench Precision bag and it is very light, but not quite as rigid as I like. It is also something else in your bag to carry.
Do you have to carry a beanie, fleece, gator, etc? If so, this may fit into a small stuff sack and be the same.
I have a sleeping bag liner, that is about the size of a rear bag. It would work too.