I’ve used inexpensive tomato cages. Some I cut the ’legs’ off of & some I’ve cut down or opened up a side to stretch open wider. They disappear as plants grow through them. I’ve used for hardy fuchsia that get floppy in rain with weight of flowers. I put them around plants as soon as the plants have some growth on them. These are established plants getting @ 2 -3’ tall. They stay in place very well as branches grow through the layers of the cages. Irises are pretty upright but might work if sides of cage cut open & threaded through the iris.

I have to admit I am confused by flopping Siberian iris foliage :-) IME, it stays upright until I finally cut it back sometime in winter to clear the way for the fresh new growth. I used to be a bit of a beardless iris geek in my younger days so have grown a lot of different cultivars. None were ever floppy.

Be that as it may, have you considered crumpled chicken wire? Comes in different gauges and different widths and is cheap. And easy to crumple into whatever shape you need. I'd think maybe just a simple arch at the proper height would work....? The new leaves in spring will grow right through it.

Update: I talked DH into making the copper supports for me. He came home from Home Depot and I promptly sent him back to return the supplies once I saw the receipt -- the cost is $$$$$. It was quite a bit cheaper to buy grow-thru plant supports through Gardener's Supply because I need so many.

Yikes on the cost and that DH didn’t question it. lol! I also wonder why you need to support your Siberian iris. I have a big clump of oregano that can get flopped around in storms so I use small branches or basketry willow cut to whatever height I need, then I stick them in the ground, spaced evenly around the plant. Then I string acrylic yarn from stake to stake around the plant to create a custom cage-like container. I use dark aqua or green yarn so it blends in. It always works great for me.

I’m going to do this in Spring for my peonies: