Any steel merchant will supply mild steel or corten steel in sheet form to any width you specify. Don't be hung up on form boss, it's just a refined use of a common material.

I would go with the same style railing as the front of the house. Place the fence across (with a gate in it) after the second pillar, between the exterior light and the window if you can. You might need to make a larger "platform" with perhaps decking, to accommodate fence swing. If you use decking it might look nicer if you drop a step from the tiles. Possibly could extend your tiles along a bit and have gate sit on the tiles. Your existing wooden steps could then be put in or, do a couple of steps again, more of a platform, then more steps so you are spreading the distance down towards your back yard. If you hinge the gate behind the pillar that may look nicer and swing away from house towards the side fence and not to get in the way of the window (if window is an issue possibly a sliding gate?).

A simple redwood batten gate that looks good too. Her toasted tea cake is beyond Fantastic and she makes a lovely cup of tea too!!

Does anyone know of any companies within Australia, other than Sidcon, that make a sliding turning gate please (similar to attached photo). Thanks.