I am further going to muddy the mix. Mind you this option isn't for everyone as needs may vary, but the idea is useful to my mind. I see you are going to do a fair amount of the fabrication, not sure what your skill sets are but .. building a trailer is a pretty straightforward task and not difficult if you take your time. When I needed a new Log Wagon
to haul my 4 foot firewood out of the bush, I was kinda bereft of extra coin so I had to do things on a shoestring budget. Course, that is usually the way I do things ... and even though I have an improved budget, I still tend to do things using reclaimed, recycled, reusable materials that fit the project needs. Remember that I am not a mechanic or a mechanical draughtsman, I is a cabinet maker ..
So if I can do this, so can anyone else who is a tad handy.
My Log Wagon is easily adaptable to a wagon that you need. Instead of the taller posts, make a bed that rests on the structural beams and add seats to it as well as sides as pictured in your sketch above. Nice thing about this design is that the hitch and tongue are the same height as the Cub's drawbar which makes for a better and safer pull. I think I have all of around $100.00 in it. The tires are off of snow blowers and other yard equipment, the 4x4's are not expensive .. about $0.35/foot, and the steel I had laying around. All that is in the narrative.
Granted there is no tie rods or drag links to make steering real smooth, but it is just a trailer after all and doesn't need all them thar bells and whistles. I have placed some sheet goods on the bed and my kids and their friends enjoyed a hay ride. At the speeds that a Cub travels the finer things are not needed. This could end up being far easier on the budget. The wood is mostly pressure treat finished with some of Mr. Irving's finest SAE10W30wt that I had hanging around. For a people mover I would use a good Water Seal like Thompson's or such. This wagon was built 11 years ago and it is still in top notch shape, a couple additional coats of Mr. Irving's Finest keeps it in good shape. I try not to leave it out doors, but ..... no matter how big you build a barn, it simply is never big enough
Now if you really want to buy one, then I would be leaning toward this one, mostly because of the look. It fits a Cub and a Hayride image to me:
Most of them are really over sized for a Cub. The one that would qualify and one that I have looked at very closely at my local Princess Auto is the identical one that Jim identified at Northern Tool.