Simply to make and very slim......
Step 1: Materials
A old power bank(2000mAH) with low battery(used),new battery(slim)lithium with big capacity(4000 mAH).
Step 2: The Job!!
Take out alls composants of your power bank,cute old battery wires and replace with new battery and connect the electronic circuit with usb parts.Cut the container(aluminium in may case) in two parts and use only one(calculate dimension you needed) and replace inside all electronics circuit and close with hot glue.
Some plastic materials used for cover the new battery(like fake leather...).
the charger isn't locked on the battery with glue,it's important;the charger give hot temperature when is in charger
state.....and you can place alls parts in different mode.
Use it with a Gps,smartphones,Mp3 or other like USB lamp every where.....You need 1A 5Volt(usb) to charge the battery in 4 hours
Thank you to watch......enjoy