Power bank using again cigarette lighter type usb charger. There are 2 versions, round and more bulky, all other dimensions are same. Added versions with external 5mm LED hole. What you need: -Plastic Battery Storage Case Box Holder For 2 x 18650 -rocker switch 9x13 (smaller one) -suitable usb charger with LED, like shown on the images. Unfortunately I can't tell what is good and what's not. Most this type chargers keeps inside basic regulators and using them with under voltage should be just fine, but remember source voltage must be at least 1.5V higher than output ( 5V). Connecting wires should be easy task, simple solder wires directly to charger metal contacts, no need for anything clever things and remember to connect switch to complete circuit. Other solution is hack car charger to pieces and use only usb connector with led and solder directly 5V regulator to connector, 7805 should do the trick. What I like to enhance is find somewhere display to show how much power is left. It would be really nice to show 0-100% and connecting this display behind push button for fast reading. Still looking suitable measurement display. If you don't know what you are doing, don't do it!