There are precious few battery packs out there with USB-C power delivery, and this 30,000mAh model from Aukey is one of the most affordable we’ve seen.
In addition to the two standard USB output ports, the USB-C port works as both an input and high-speed output port that can charge a 12" MacBook at full speed, or raise your Nintendo Switch’s battery level while playing it at full brightness. It’s pretty huge, but 30,000mAh should be enough juice to get you through even the longest flights. Just be sure to use promo code AUKEYPB6 at checkout to get it for $54.
Update: So it seems that this might technically run afoul of TSA guidelines (though there are exceptions when airlines give permission), but honestly, I can’t imagine it being an issue.
While you’re at it, here are two USB-C to C cables for $5.