I've used the Yakima Basket Case on my Thule rack for many years. It's helped extend the capacity of my XJ, a not-too-big vehicle. It's a simple item, and has survived a lot of hard use on my 1988 Cherokee.
I thought about using three rails and two basket cases, and I think the post above showing it in practice looks super. For not an awful lot of money one gets a flexible and transferable racking system.
I've seen the ARB roof racks, and they seem strong. I've liked all the stuff I've gotten from ARB over the years, but these are too big, and too specialized for my taste.
It's common for utility vehicle racks in Europe to have rollers on the back edge, to aid stuff going on and off. I've seen it here in the US, but it's less common. I like the idea where longer items need to be handled, especially solo.