The set-up that moolie posted a picture of looks useful, as a way of starting to visualize something. I'd think about some kind of strapping (1x4 boards, or whatever) underneath, then sitting atop that a layer of stucco wire (or something similar), and then above this something like landscape cloth (or Tyvec with punched holes). The stucco wire has some body to it - and this way, with the strapping and the stucco wire, you'd have some support for the cloth-like material that will hold the soil in. But it will still drain, which you'd want.
You'd have to replace wood 1x4 every few years, because due to contact with soil suspended in drain water, it's going to slowly rot. Flat steel would last longer, though obviously would rust. Flat aluminum stock could be good, if you could find it.
Possibly you could find the understructure reinforcement, the stucco wire, and the landscape cloth from salvage. Still, in the quantity you're likely to require, I don't think it would be a substantial investment to acquire these materials new (if you bought 1x4 for the strapping). But only you know what you can afford.