buy this set:
https://www.amazon.com/Webbing-Reflective-Polypropylene-Release-Including/dp/B08P1R2WNR/
OR
https://www.amazon.com/Plastic-Buckles-Webbing-Backpack-Repairing/dp/B09FJDDC5W
You can use some of the webbing to make tabs and sew it on the opposite corners/edges of the bag or directly opposite; sew it onto the bag with an X and box shape or use rivets. Make the adjustable strap.