A flat cover or one with some height to it?
This recent thread had some good pictures and info of an old (store-bought) canvas top with simple framework that you could duplicate or improve upon. https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1389782/canvas-bed-cover.html
Some of the aftermarket flat roll up aluminum covers are based on rolling hurricane shutters, or so I've been told. If so then perhaps you could adapt one the right size (probably made to order) to your truck even if the aftermarket truck bed versions aren't sold for 9' beds.
I've seen old aluminum truck caps for the AD trucks but not sure I've seen a 9' version, and rarely see them for sale at any length. I've been thinking about making my own one day with aluminum angle for the framework and aluminum sheet riveted to it, similar to airplane and/or airstream camper construction methods. I have a collection of pictures of original ones to guide my own design. One detail I like is a fold down cot frame on each side, making it a real basic/simple camper.