Metal Slim 4000 mAh (with White Box)
 
A small power bank that has a classy and sleek look will definitely feel like a cool phone accessory to have, with it, you won’t have worry about running out of power, it can be fit into your pockets or in a handbag easily and provides the capacity of 4000 mAh which will sound a lot better than running on low power and having to spend rest of your commute staring at things. You can even think about giving it as a promo gift as it can be printed with a logo or brand name.