cardboard might not be such a good idea. If you plan on doing multiple grows it could get moldy, and i dont think it would hold the smell in very well when you want to ventilate it. If you want a good grow box your looking atleast 80 bucks. There are some good grow tents out there or you could go to ikea and get a wardrobe there for pretty cheap. Just remember that you will most likely be putting ducting and heavy fans in the side of that thing so the material has to be strong enought to support said fans. Of course if security and stealth is not a big deal for you that you could probably use a cardboard box without to many probs. Just make sure its light proof when they flower.