That was another issue I had, out back on the gas pipeline clearing it was at a pretty good slant and the whole frame of the cart would twist. I just went with a ball hitch and figured that would provide enough rotation, but looks like yours has a good solution.
Was just going to get the 1,000# hubs but they were out of stock and these were only a few bucks more. Kind of overkill but hey it should never fail! 2" ball so I can put it on tractor or truck, etc.
Used a couple pieces of 1.5" angle steel (3/16 if I remember) for all the framing, and got one piece of 2" square I used for the axle and tongue. 5/4" PT deck boards for the bottom and removable tailgate, and just some scrap PT plywood for the sides.
I used a set of cub cadet rear wheels I had, so they are nice and wide 23x10.5x12
Was a lot of work throwing it all together, but well worth it for me and a fun project. There were several aspects of it I designed specifically for my purposes, so nothing you could buy over the shelf came close for what I needed. I spent a little more, yes, but I think I got A LOT more out of it. Only thing was I wish I had a bigger welder! Welds are good enough I think, but I wouldnt trust em on the road. So far bouncing heavy loads over ruts and groundhog holes I haven't had any failures yet.