Don't use copper. It's far too soft. And solder is too brittle. It's not structural.
Steel is your friend here. It's cheap and available in countless shapes and sizes. Plastikote spray paint will stop it rusting.
It's a shame you're not closer to me. I'd of let you come to my workshop and I would of welded them up for you with all my posh new welding equipment.
back in 2009, I used flat bar for my racks which lasted me all through Africa and lots of tumbles. I over engineered them.
If you're determined to do it yourself and don't want the hasel of bending tube then I suggest buying some Mild steel flat bar (also known as black steel). DON'T by bright steel. It's too brittle.
Look at about 30mm - 40mm wide and 3mm thick. You can buy a 'length' for about £20 which is 7m long
look at my DRZ prep page on my website for some pictures of mine. I made those in 2009 before I knew how to weld. Using a £30 welder and some scrap flat bar which was way too thick.. 5mm I think. TOO THICK.
They took me about an hour to make using a vice and a hammer and a propane torch.. They look SHIT but were stronger than the subframe of my bike haha.
Let me know if you need anymore help. I'm doing LOTS of fabrication at the moment.
Here's one picture. I'm ashamed to share it due to the poor quality of manufacture. However, I did make them in an hour out of scrap metal whilst enjoying a
2001 NX650 Dominator.
EX - BMW Dealer Technician