A cardboard box works quite well and the price is right @ free.
I second the Milk crate, or plastic crate from Target. It doesn't move around, and holds thinks upright. I also removed one plastic carpet clip on each side, and replaced with a "J" style hook from Home Depot. Nuts, Washers, and two "J Hooks". I can hook two bags on the side with the factory bag hook, and another bag on the other side.I have three milk crates bungee'ed in to the back of the trunk against the seats. I can also get to them from the cabin if needed.