Let me know how you get on with this! A battery pack would be a very very useful thing for the Switch, however, I've heard some very mixed things about using the TypeC cables. Most of the consensus was that while the Switch uses a TypeC cable, it doesn't provide enough power in order to charge it (the standard USB end on the other side being its limitation. There was a very good video about this I saw on YouTube on a channel called TheKnow, where they tested this out.
Unfortunately, what they found out was that only the official Nintendo Power Supply (the one with the home socket) was the only thing that could actually charge the switch during play. What they discovered was that while the switch does use any standard TypeC cable (and they do work for powering your switch), the problem is when charging during play, so you could only charge it with it switched off (and even then it took many hours longer than it should). Basically what it all boiled down to was that it is the power supply section that gives it the power it needs, which is why TypeC cables alone can't handle it (part of the fact being that it's standard USB at the other end, but also partly because there is now power unit to ampify the power to give it what it needs). They did specifically mention that it was useless from any of the power bank portable battery chargers currently on the market because of this. There was one they found which uses TypeC to TypeC on the other end and was more powerful than usual, but it was pretty bulky, and even that took longer to charge than it does from the home socket. (which is a real shame, because a portable battery pack would be so freaking useful if they worked like they do for smartphones).
If anyone made one, I would personally buy one like a shot (and I'm sure it would be possible somehow, just no-one seems to do one that works in this context at the moment) However, let me know how you get on with yours when you finally get to try the one you ordered, and if you get it to work! Be sure to test out what you find and post back on here just in case you've managed to find one that has solved this problem. (But I wouldn't count my chickens just yet, as all the things I've heard so far suggest that it can't yet be done!) -Though I will happily be proven wrong, and you will indeed be my favorite person if you're the first to help me get one that does work properly!