I'm making a stage out of a cardboard box! I need some advice

[Petzbreeder]

I'm not sure if I posted this in the right place.

I'm working on making a stage out of a cardboard box at my art group. I plan to record myself with a green background and edit it out (Chroma key) so that it looks like I'm inside my box.

The problem I have with my box is that the top and bottom have started bending out of shape, so my box isn't completely flat anymore.

Will gluing another layer of cardboard on the top and bottom permanently straighten it out, or will it still bend out of shape again eventually? If that doesn't work, is there anything else I could try?

I need it to be permanently completely flat so it doesn't look all wonky when I use it as a background.

I'm going to add some tiles to the bottom of it so that it looks like a dance floor!