The simplest way to improvise a backpack to be used when none is available for whatever reason, from a survival situation to just an afternoon flower-fetching stroll, just takes the use of a blouse, jacket, pullover or whatever with two sleeves and a zipper or buttons.
Loading wood sticks: A couple of jackets are being used to carry the combustible material, leaving both hands free for other tasks.
It is really something so simple that it needs scant explanation, but it is nevertheless good to know about it. You first start by taking this garment out - of course - and then buttoning or zipping it up. Join together the sleeves, forming a continuous arc that will act as the harness of the expedient backpack. Close its lower part and the neck using buttons or some rope or cords, and then fid the way in which you can carry the whole contraption on your back by passing the joined sleeves around your shoulders.
ººThis is a very practical method to carry things that you didn't think that you would find or rather voluminous objects that cannot be placed normally inside a backpack, like wooden logs to be used as combustible for your campfire.