I can use a string backpack for a couple of hours, which is what I do. We pack ours with lunch & a frozen water bottle. I carry it until noon, eat lunch and then fold it up & put in a small crossbody bag that holds my cell phone, sunscreen, lip balm, sunglasses, passes & cash for the rest of the day. Works perfectly for my family.
I wouldn't carry one all day though. It's painful after a while.