Oh, right. That.
Someone is working on a huge project here for the world's fair next year. They've been air pruning a forest of trees for the past 5-7 years, so that when they finally install them at full size, they'll go wild because of the huge number of roots they have.
Have you thought of that hanging pot thing with Coco liner?
http://www.selectiveoutdoorsupplies.com.au/Uploads/Images/hanging-basket.jpg
I imagine just about any tough fabric would do for a season. When the season's over,
compost it. You could hang it with a rope.
Burlap sacks might work well and you can usually get them for cheap. If you sew one end, you could get two pots for the price of one burlap sack.
If you're looking for something in the cheap metal department for a container, you might try
chicken wire that you mold around a container. It's usually pretty cheap that.
Just brainstorming ideas here.
If you need something smaller, you might go with something like orange (
fruit stands, supermarket supplier)-fishnet, although you might find yourself in a mess of plastic near the end. Or burlap sacks cut up into small circles, placed in a pot, allowed to settle in, and then taken out and tied to keep it together.
Let me know if I'm getting close
William