where to find plastic to keep things leather hard?

hi everyone! i’m currently in the process of setting up my home studio, and i was wondering if y’all had some tips on where to source plastic sheets for keeping my pieces leather hard? i have some old vacuum seal bags for clothing storage i was maybe going to try out, but they are a little less flexible than i would like. i’m worried they’ll damage my pieces.

any tips would be so appreciated! 🥲

edit: thank you all so much for the ideas! y’all are the best 💙