My Idea for an improvised backpack would use a pair of old pants and some rope

Cargo pants or BDU pants work well as they will have external pockets you can use for smaller things

the idea is to make a pack similar to a Veshmeshock http://wp.me/p1Ff3c-h3

Take the pants and lay them out legs facing down

tie the legs together and closed with a small piece of rope

Tie another longer rope that will form the straps to the  first small piece with a large gap

Fill with stuff …

tie top like Vesh ….

and now you have rope and a spare pair of pants for no extra weight