The aluminium bar worked out real well; I did have a problem initially getting the tight loop tight enough - the 5mm thickness makes that tricky (here I'm trying to vice it, no dice):
Took a lot of work including hammering (but I don't have a workshop or even a proper work bench, if you have the gear it's probably easier). The larger round bend though is easy, you can just stand on it and bend by hand, nipping and tucking as you go. The finished article is as strong as you want it, it doesn't flex/bounce at all.
Here's a teaser pic from earlier today (I'm saving the rest until the camera is mounted & balanced):