Box Materials

So after watching Philips box tutorial a few times, I really want to make a small box myself. My plan is to use some thin leather, and therefore, can't use a thick wood

(would not be able to stick through) So searching around, found some other materials. But cant source them anywhere!

This looks like bleached greybeard, but craftsman says not.

Apparently he found it in his printer😆 This has to be translation error of somekind right? Build like a Lettuce huh?

This box was made only using this stuff! Credit -----> unbearable_lightness

This looks really good, but can´t fint it. Seems it used for shoe soles? (credit go_leathers)

So I need your help. Anyone now some alternativ to wood I can use? Something I can stitch through, but with enough rigidity to be a box 💪🏻