3D print a replacement backpack strap adjuster! I recommend printing from a strong filament such as ASA or nylon. There is a model file for a thin version and a (barely) thicker version. Both should be plenty strong for an average backpack load. It should work for any strap 20mm wide or narrower. You could probably scale the width in your slicer if you need wider straps though!

Sorry I don't have photos currently. I'll post them when I can (if I remember haha)