A bag should never be weightier than its contents. Taking this idea to heart, German label VANOOK crafts its backpacks in a coated cotton fabric that was originally designed for tents, making them especially lightweight and water resistant.
Here is the pattern for the faux leather rucksack I made. (add a waist belt?) You can see the scale at the bottom right. If you make sure the red line is 10 cm when you print out the pattern, all the pieces will be th.