Old large damaged rucksack - to make strimmer harness (Forest Hill OX33)

Hi, I'm looking for an old, large size rucksack - that I can adapt to make a shoulder harness for a petrol strimmer. So ideally a rucksack that is too torn / ripped etc. to still be used as a rucksack. Only really need the shoulder straps, and the 'back rest' part of the rucksack - so if the rest is damaged that doesn't matter. Thanks very much.