I just used a steel caster from Northern Tool. Haven't used it yet, so I don't know if it'll work. I radiused the edges a bit so they are less likely to dig in, kind of like a lower English Wheel roller. 2" x 1/4" box tubing to fit into a standard receiver hitch. 2 holes in the tubing allow it to store on the truck while pushed in, out of the way.
