Wow, this conversation brings back some serious memories! Back in the late 80's I was mfging backpacks for a private label out of Cali... anyway, we were the first to introduce a vacuum formed/thermo formed lexan sheet for the frame sheet. It was pretty much indestructible, but was a tad heavy, expensive, and very difficult to work with. We actually started with kydex, which is the material that is used often in industrial bldgs as interior corner guards... anyway, kydex can't handle the cold, and with any flexing or impact will just shatter.
Lexan is a great material, and that old private label is still mfging that product. Thanks for the stroll down memory lane!