VC, I think that battery box on the cart with the RC winch is one of the most creative solutions I've seen. great idea! especially with the fairhead directing the cable onto the spool. loading and unloading effortlessly.
Look for TAP Plastics or a similar supplier in your area. They can sell you everything you will need to build your own, or they can build it for you. Working with plastic sheet goods and liquid instant welding is actually quite easy.