rucksacks/backpacks (Oakenholt CH6)

I'm looking for unwanted rucksacks/backpacks, sort of 25-40litre size, especially if your child has now changed from one fad to another and they are too cool for school :) This is for hi-viz parkrun volunteers to be able to carry essential kit around the forthcoming Flint Castle parkrun. Doesn't matter if worn or not looking brand spanking new - they'll be put down on the ground anyway. Thanks so much :)