Following my recent folding bike build I have the need for a bag to transport it in planes. I used a full sized padded bike bag when I flew to Spain but now I'm home and have more time, I have ordered a custom made bag for future use. What I'd like, is to line the bag with a lightweight, flexible hard plastic (or somesuch), sheet that I can cut to size. I used hardboard as a stopgap this time but it isn't great and warps pretty quickly. As light a material as poss, with decent strength. Any ideas?