I did this with a more or less rectangular plastic trash can. I put a regular draw bar through it, and had a bunch of re-bar lying about that I bent up and poked through the draw bar holes. I had an attempt at a third arm for my back hoe that I used for the 3rd point of the hitch. As I filled it with concrete it swelled out - I put in 6 80lb bags of concrete, and 2 bags of mortar. This ended up weighing in at about 600 lbs! And it works GREAT!
Since I built it I have trimmed the iron, and cleaned it up. It will probably never be a beauty, but it works like one...
One gentleman used a 55 gal drum, and members figured it for about 1100 lbs!
As Eric McCarthy said, there are many threads about ballast boxes here, with as many good ideas.