You need a fast food container, the sort you get take-away Chinese or Indian food in.

Look at the bottom. It is made of aluminium. Once this aluminium was a flat sheet, but it has been pressed into a tray shape. This is a flexible material.

Look at the cardboard lid. You can bend it up a bit, but it will flap down again. The lid is not flexible, it is elastic.

Now close the container by bending over the lip of the tray. Again, the aluminium stays in the shape you bent it, holding the lid fast.

We use many materials like this. Can you find other flexible materials used for fastening things?