Thank you Alison, John, and Joe!
I've been away working for the last few weeks and unfortunately won't get a chance to start building for a few more. The upside is that I get to obsess over details in my free time!
Joe, good advice on layering the gloves/mitts. I have a similar approach, though I tend to have dedicated gloves for getting sweaty going up and then getting colder skiing down. Making the micro switch glove friendly is an excellent idea, I might have to try that once i get more comfortable with morse. I'm new enough that I need a comfortable, low-pressure environment to send/receive comfortably and that means summer. I have a QCX that I've been playing with just for that purpose. I even modified it to draw 75 mA on receive!
John, thanks for the link! I went with the 100x100x44mm box for the extra depth (not much, admittedly, but enough to provide a bit of wiggle room with the batteries.) It's arrived from China already and it looks like it will be small and light enough to be worth bringing on multi-day trips, which I'm very excited about! I really like this idea of designing boards to fit inside these enclosures and there seems to be enough variety that designers have don't feel too much pressure to conform to any one standard.
Alison, informative as ever! Thanks so much for the information about the TCXO and also the 817, that had been bothering me for a while. There's an old trick SOTA operators in the UK have been doing for years to keep their radios splash proof, and that's to use a clear disposable shower cap placed over top of the radio while operating. I might start there. This enclosure seems small enough to fit in my jacket pocket so I'll be able to add that to my list of ways to keep the batteries warm.
Through some extensive, and somewhat random, browsing on wikipedia I discovered Lithium-Titanate batteries, which have a temperature operating range down to -30C in addition to higher charge/discharge rates, greater cycle life, and being much more environmentally friendly than other lithium chemistries. The downside is that they have a lower energy density and inherent voltage (2.4v/cell) than lithium ion cells, though still better than nickle chemistries. They are also more expensive at $13 for a 1300mAh 18650 cell from battery space (link: https://www.batteryspace.com/Lithium-Titanate-Battery.aspx ). I may or may not explore this option at a later date. For now it's just fun to think about.
Thanks again to everyone, I'll keep posting developments as they happen!