External batteries don't have to look good. They're big chunks of lithium-polymer wrapped in plastic and USB ports. But there's no reason that they shouldn't! Check out this Aukey 12000mAh battery, for example: nothing special, nothing fancy, just clean lines, a black case, and an obvious purpose. If you squint it kind of looks like an obelisk from 2001. Today Amazon has a coupon that drops the already-reasonable $24.99 price down to $17.99 - that's just .15 cents per milliamp-hour!
12KmAh should be enough to charge even the biggest phones three times over, with a bit of a margin for the inevitable efficiency drain. This model has two five volt, two amp USB ports and a bonus LED flashlight, which is handy for when you inevitably drop your USB cable underneath the airline seat. It doesn't feature any fast charging tech, either going in or coming out.
To get that price (complete with free two-day shipping for Amazon Prime customers), enter the coupon code "TEBRJI47" at the checkout window to take seven bucks off.