You're going to need to make friends with the local welding/metal fab shop.
Aluminum square tube. They'll put their 12' lengths in a roller/bender to achieve the desired radius. Then they'll cut and weld in square tube uprights. Then you're left to either skin it yourself or have them skin it. That should give you a blank slate to work with. Also remember, you need to think about how you're going to mount this thing BEFORE you have it fabricated. It's easy for them to weld on some legs and a mounting plate when they start the fabrication process. Adding mounting points after can be more difficult.
Also some people might build this in sections. You can simply bolt them together on site. It all depends on what you want to do.