They're not going anywhere right now, and that's only held on with 4 screws in the far corners, nothing in the middle. This cardboard came from a box that car body panels shipped in, so its a little tougher than you're average box. But, that's also why I don't think it is a good idea for long-term usage...

Looking into fiber board panels with a wood frame. That doesn't look like it'll cost too much, and would be more durable than the cardboard box.