You can make a wooden growing box very cheaply, and to whatever size you want. The main attribute of a wooden growing box is drainage. By leaving spaces between the bottom slats and covering them with wire mesh or something, you get wonderful drainage. Which means good roots.
Here is a link to one I built. If you search for "growing box" you will find a lot of information.
It's a good idea, and you will be pleased with the growth!
Joanie