Look at this! What a great idea! I completely want to do this. I bet I could make my own bags, perhaps start with the salad garden size, and add handles to make it easy to transport inside if the weather threatens.
The kids are off school this next week - maybe I can find a few minutes to work up a prototype. These are made of (I think) landscape felt (which I have) and I suspect hold their form by way of the potting mix and root mass of the plants inside.
If the prototype works, I'm definitely going to make some of the potato bag size. And if they don't work out, Gardener's Supply is about 1/2 hour away.