100 hp Tow-Behind Rock Picker

I need a robust tow-behind rock picker that my 100 hp tractor can handle comfortably in broadacre agricultural fields. The machine must reliably lift and collect medium-sized stones—roughly 3 in to 6 in... (Budget: $250 - $750 AUD, Jobs: 3D CAD, Autodesk Inventor, CAD/CAM, Engineering, Machinery, Mechanical Engineering, Product Design, Solidworks, Welding)