I recommend a Ortlieb Quick Rack LINK and a pannier bag or you could repurpose some other type bag but would have to customize the attachment to the rack. After setting up the rack, it takes 1 minute to attach the rack to the bike! Because it is so easy and fast to attach and remove, it is does not need to stay on the bike. Though permanent rear racks are less expensive, so you pay for the portability. And carrying things in a pannier, on a rear rack, is very difference than carrying groceries in a backpack, which I did last night, coincidentally.