Portable battery packs are nice. When they're slim, they're even nicer. Taking up less space goes hand in hand with the whole being portable thing.
iMarku makes a 20,000mAh external battery pack that is only 12mm thick. You can currently snag it on Amazon for $35.99.
By using the coupon code MCMMHJJH, you can make that $19.00. Alternatively you can use ZVSRSDM5 to bring the battery down to $19.99, but that's an extra 99 cents. Save that in your pocket in case the first one expires, I guess, or use it if you plan on ordering more than one. Though if you're planning on ordering in bulk, there are other coupon codes listed on the page to consider. A789V27X, for example, will save you 55% if you order 3 or more.
This battery is thin, but it's still a little wide. That makes it something you might find more comfortably squeezed between your laptop and a textbook in a messenger bag than flush against the side of your hip. But that's your decision to make. You can find the link to Amazon below.