got a picture or something of a 'rock box'?
If you've got scrap and the wherewithal to put together a rock box, that's a cheap and versatile option as well. You can put rocks, scrap, or sandbags in it when you've got something on the back, and empty it when you don't. It can also be used to carry chains, splitting mauls, and other things that may come in handy and to pick rocks that you may come across.