Get a barrel or any watertight container in a size that is approrriate, a barrel works best. Build a frame for it to set in that will connect up to 3 point hitch. You probably will want to weld the frame to the container. Tap a water spigot to the bottom of the container. Fill it with water for weight. If you want to leave the water in it, add anti-freeze. What really is best though, drain the water by the spigot when you don't need it before disconnecting from the tractor. The weight box, barrel, etc. will then be light enough that you can easily move it around by hand when it is not on the tractor. When you need to use it again, just fill it up with water again after it is attached to the tractor. A barrel really works good because with the top still on it, you can fill it through the bung hole, then drain it through the spigot at the bottom. Some barrels already have a spigot. Easy on, easy off, easy to handle.