All my suspend  aviaries are built out 25mm box galvanized steel. I also use zinc flat sheet metal anything above 0.6mm otherwise its to flimsy and makes your job a lot harder. Instead of using Avery wire that you buy on a roll I use sheet mesh, it comes in sheet size of 1200mmx2000mm and the holes are 25mm square. The mesh thickness of mesh is 2.2mm, this mesh is very strong and the Alexandrines wont bend it or break it. I use zinc coated hexagonal metal thread Tec screws for fixing mesh, roof sheeting and wall sheeting. I cut all the steel on a metal drop saw, so that my cuts are even and I can cut two lengths at once. Some things to remember are that always make your cage size to standard wire and sheet metal sizes {900mm,1200mm and 1800mm}. Draw a sketch of your cage and write the measurements down so you have a cutting list and you don’t have a excess or not enough material. I also brace all my legs, to stop swaying and bending.  All my locks and hinges are galvanized to so rust wont be a problem. I weld all my joints with a stick welder but if you know someone with a mig welder, its a lot quicker and a better weld quality. There are also plastic joiners these days that you pop rivet the corners in with. The other option with suspended cages is to have a walk though at the back of the cage, this is a cage joined to the back of main cage to stop birds escaping from you that might slip past your suspend cages doors. I don’t have walk through as I don’t have enough room, but if I had the room I would. If you lose one dear bird, the walk though would have saved you money and you wouldn’t be annoyed with yourself.