The aluminum ramps on rental trucks can be tippy when loading heavy objects with higher center's of gravity. You'd do better with a lift gate, but unless you have experience using one, I wouldn't recommend starting with a large toolbox. They can roll off while lifting. Your best bet would be to get an enclosed trailer with a ramp. The grade on the ramp would be lower, and the ramp would be wider. Less chance of a mishap.
Whatever you use, spend the money on good straps to secure the box from rolling around. Don't use cheap 1" straps on something like that. Use padding against the wall, and on each corner under the straps.
If it was only a few miles away then an open trailer could work if you are careful. But for longer distances, an enclosed box would be better to transport it.