I noticed one time when I replaced windshield wiper blades that some of them have little metal strips embedded in them. I pulled out one of those strips to save and just found it in my garage. It looks very similar to the material in Estes engine hooks. I measured an engine hook at 0.128" wide x 0.024" thick, and the piece from an old windshield wiper blade measured 0.160" x 0.029" thick. It is 50% heavier than the Estes hook but also stiffer and stronger. If used for D and larger motors the weight wouldn't be as much of a penalty and they could be formed to whatever length you needed if you wanted to use them for an Aerotech single use motor for instance.