Saw one of these yesterday and thought it would make a great project. Looks like a few other people have built them too.
I haven't built one yet @timjeffries but would like to. We do actually have some leftover reo at our place at the moment after some paving work (over a concrete slab) so I thought it was a fantastic idea. I saw one and thought it was a really original idea. I was surprised when I googled that lots of people seem to have done it.
Bunnings has a similar project on their D.I.Y. advice section - D.I.Y. wire vertical garden. I'll also attach the build video below.