1) Wrist Bands *2 or Rubber Bands of good quality which will not break easily.
Now take the wrist band and place at the edges of the smartphone look at the image for placement.
This will work for most of the power banks as long it is not too big or too small as most of the power banks nowadays are of 10300 mah which approximately of the size which i have in my project.
If you have a slightly biggered wristband or rubber band you can slide a pencil or a marker at the back and make a stand so you can watch movies and also charge your phone.