About ten years ago I bought a bender from a vendor on EBay, for producing a 20' diameter hoop. Using 3 pcs of the top rail you refer to. He had worked the arc of the bender out just right. I just took a look at it and, no he has no name on it. But I think he sent a cd with it, If you need that I can try to find it. I did mine 28' long, 2' OC and covered with corrugated polycarbonate. Wound up with more leaks than I wanted due to my bad lap system, so I overlaid 7/16 OSB on the top 1/3, installed to the tubes with self tapping screws. Then I shingled the osb. So, almost exactly what you envision. The osb bends to this gentle arc fine, if laid lengthwise and staggered. Obviously would not conform if laid the opposite way. Still cheaper than ply, and fine if you cover. If you want to go metal like corrugated, you will have to lay lengthwise also, and you will get leaks unless you can obtain sheets the full length of your building. ( it is the end over end joints that will leak) You can lap the lengthwise joints enough to prevent leaks there. You may be able to get full length sheets from a real steel supplier, not big box.
BTW the link you sent is just a fabric building, there are tons of those online, such as Clear Span etc. Most of quality are to large for you though.