What you describe is available commercially, called 'D-shaped tubing' or 'half oval tubing'. That does not help you to make it but might give you a source to buy it.
How long do you need it? Could you pull two short 90 bends on a flat strip and then press into a U-shaped bottom die using a thin vertical web and round bar as a top tool? It would not close the section and you'd need to slide it sideways off the top tool to remove from the press. Then it is only a single central seam to weld up.
bought myself a little RE classic 350 and I'm going to make my own rack/backrest for as I can't find a suitable one. they're all to low.
the picture isn't my bike but getting the same colour etc. you'll notice the fixing.
I've got some 16 mm solid bars so I'll get them machined to fit inside the tubing and dress the bar to suit.
recently I bought a W800 and I'm not really keen on so I'll sell it on.
the 350 is ideal to plod along on our local moors. no rush to get any where.
cheap to buy,tax,ins plus 100 mpg so what not to like?