Have done a lot of this using a Rigid brand ratchet 5/8" tube bender. Used to be able to go to used ski gear sales and pick up lots a raw material cheap, but now the poles are all made of an alloy that is too stiff to bend - so it shatters instead. Unfortunately, the Rigid benders now go for several hundred dollars - mine was purchased long ago for <$100. There are cheaper ones from the Tool order companies, but they are of much lower quality, & don't ratchet.
Recently made a LaFuma style folding camp chair, tho. Found a hunter's folding stool at a Brookstone outlet here in NH with alloy that looked bendable, but it was 3/4" inch tubing. So bought a cheap mail order bender for under $100 since only 4 bends were needed. It worked, but only with much more care and much less facility than the Rigid, and I had to bolt the main bending plate to a wooden post in my basement. The bender kit weighs a ton, but handles a number of different diameters of tubing.
The earlier post about Quest tubing might be a good source of tubing. But don't think anything less than 5/8" tubing will be robust enough to hold its shape unless you really baby it. Maybe you can borrow a Rigid from a machine shop just for your 2 bends, or have them do it.
A suggestion: Invert the U, and buckle a hip belt band to the lower legs. You may need a bent cross piece at the bottom of the U for enough tension on the band. Attach the hip belt to the band at points not too far from the band's center, so that as you walk, the belt can rotate a little with your hips. You can put cups on the leg bottoms to take some of the abrasion when you set down the pack, and the frame top will be rounded at the bends so as not to catch on stuff, and look nicer. I think that is the essential design of the frame on the new pack BPL is selling - lots of pix on this site.
Have made a lot of fancy bent tube frames for panel loaders and the like, but for lowest weight, finally settled on old reliable, the top loading pack, but with a more flexible ABS tube frame suppporting a stretched mesh back panel. (Design marketed by Alpine Designs years ago). But you could probably do the same thing with the inverted alloy U.
Have found stretched mesh to be the most comfortable back panel design.
Hope this is helpful.