Regarding

Simple single-buckle backpack strap fixing clip/buckle
There's no need to use a support structure

Installation method

1. Fold the backpack straps in half from the middle
    
2. Place it in the middle of the backpack straps
    
3. Tighten the straps on one side of the backpack
    
4. Then, tighten the straps on the other side of the backpack
    
5. It's all done