I used a cardboard box, and plastic crate, wrapped them both in leaf bags, and voila! just the right size pots!! I put the bags into the crate and the box - like liners.
I cut holes in the bottom of the bags for drainage. Cut off the excess bag length. And taped the bottom.
Before potting, I put rocks in the bottom to help with drainage. I rumpled up a brown paper bag to take up some of the space, as I thought it was more than needed for the plants I was putting in.
And there you have it - the wrapped box and crate as planters.