I have seen this done a few times:

Buy a few inexpensive large stuff sacks. Usually the nylon will be strong enough. Improve them by treating them with a durable water repellent and over-stitching the seams. Stitch a small rubber ball, wad of cloth or sturdy ties into the lower corners. If they don't have double cinch cords install them.

To use you put what you will inside and the top is drawn tight and the cords are looped at the ends and drawn tight over the objects you stitched in earlier.

The result is a simple backpack. The doubled cords don't dig in too bad but you will want to limit the load to something less than twenty pounds. I have day hiked with one of these carrying my rain poncho and liner, jacket and a few other light items without any problems. I carried the water and a few other items on my belt or