I'm looking for some high quality cases for 18650 and 21700 cells. Ideally cases that hold 4 or 6 cells per case, are waterproof, and high quality manufacture e.g. good for carrying on a backpacking trip, while mountaineering, or very frequent air travel with these cells thrown in a carry-on. I see no shortage of ~$2 amazon/google specials but not sure how to feel about these and would like something robust. Pointers welcome!