Problem with CAD programs is...
You have to learn CAD... not a walk in the park.
You still have to make adjustments for panel sizes and thickness, angle thickness, especially when working with cope joints. It still have to be finessed by the fabricator. When I had to do this, I did it in Auto-Cad, I made a template, but still had to make up for all that, and go talk to the fabricator on complicated jobs.
You can design these accurately with Corel or Illustrator with Cadtools but they would be flat drawings, not the cool 3D drawing. You can also add the attachment details for accurate alignments of baseplates/post attachments.
To get a basic 3D drawing, I have done it in CadTools using CADaxonometric in scale, or using Illustrator, work in points (72 points = 1") and extrude angle profiles.
You mentioned riveting... why rivet?
I understand maybe one panel so you could have access to the internal attachment, but all of them?