but I have some of the regular boxes that you get things shipped in, and they worked well. The boxes that copy paper come in do NOT work well....although they are surviving since I put stakes in around them to keep them from falling apart. A shoebox will never hold up from what I have seen of my boxes, but a regular box of the same size would. Or at this time of year, you may be able to use one and it would make it.
Another thing that worked well to keep the boxes in shape was taking the flaps and folding them to the outside, then taping them. That gave extra support. But all of my boxes are still surviving at some level. I did not use any plastic bags in or outside of mine, but if you do, make sure you put drainage holes in them. I don't want to put plastic bags on the inside of the boxes, because I think that it is possible that the roots may use the cardboard (or may even go through the cardboard), and get more moisture or nutrition. I will let you know if the roots did get to the ground when the season is over and I check how this all worked.