I have been looking at this -
http://www.omnicharge.co/
The Omnicharge 20 has a 5 volt / 3 amp output (15 watts) on a single USB port which is the highest I've found so far. Not sure if it'll work in practice. But I think the key is getting something with as high of a wattage output as possible.
According to my Coconut battery Mac app, a light-to-standard load (web browsing, a few light programs open, half brightness) makes the 2016 MBP 15 discharge at around 10 W. The key seems to be to find a charger that can output a wattage that matches or exceeds that. But most chargers out there top out at either 5 volts / 2.1 amps (10.5W) or 5 volts / 2.4 amps (12W) which can't keep up with the Mac if it's operating at high loads. But as altaic said, it can slow the battery drain significantly.