Polyethylene
Polyethylene in its High-Density (HD) version is a very universal plastic. It is sufficiently strong to serve as a stiffener, but still adapts to many shapes and always remains somewhat flexible. In addition, it is very robust against abrasion and very break-proof. And it can also be processed very well: Polyethylene HD can be cut with a cutter or scissors and the sheet material can also be sewn with a household sewing machine. Only when gluing there are restrictions that should be considered. Polyethylene HD sheets are very well suited as back stiffeners for backpacks, which should be somewhat flexible for an ergonomic shape. For applications where you need a stiffer and even more robust plastic, polyamide sheets are better suited.