Hard for me to know exactly what you're looking for without a little bit of a more in depth explanation (how it's going to be mounted to the square tubing, material, etc.), as I don't know a lot about hooks, but I'd say that a good bet for anything that is interestingly shaped hardware would be McMaster Carr (https://www.mcmaster.com/). It's a little expensive, but it's well made and they have a good variety of choices so even if you don't find that hook exactly, you might find something close.
Outside of online retailers, someone posted before that a local "store fixtures" supplier would be your best bet and I agree with that.
I've made somewhat similar hooks to hang extension cords and stuff on slats mounted in my backyard shed. I just bent steel flatstock using a vice, a stout hammer, and some general aggression.
Thanks everyone! McMaster didnt have what we're looking for. Luckily we have some welding equipment and we'll probably have to make our own. Thanks again!
@lwinters630 and @Diggits5Unistrut brackets as in the type where you securely fasten a length of strut vertically on a structurally solid wall then latch hooks horizontally straight out from the wall and hang a few hundred pounds on them; the type of strut brackets routinely used on the poured concrete walls of underground high voltage manholes to support loops and splices of 13k8 and 27k6 lead jacketed, or in our current times, TEK below grade HV cables??
Those types of Unistrut (or Canstrut if you're north of Donald's walls) brackets would certainly hold the weight of any lengths of "aircraft cables" you'd conceivably coil and store on them.
Toodleoo!
Ron Hebbard