I recently got one of these -- I'm not sure if it's what you're looking for but I think it's pretty neat.

It's an extremely basic backpack that is light and folds up into itself when you're not using it. I haven't used it a lot yet but it seems reasonably sturdy for the price and I really like that it collapses into a packet the size of big sandwich. It's on the small side (think elementary school kid backpack) but I think it would hold what you're describing.

It's definitely not fitted though, it's just a backpack with basic straps, so it might not be that comfy for running with.