Another thing worth carrying is an insulated bottle. I have a stainless steel one made by Liquid Solutions that cost about $20. It works really well and will keep hot tea warm all day. I made a cozy for it out of blue foam (the cheap pads you can get for $10 at Target or similar stores).

How well it would work at -30C, I have no idea, but it works well down to about +15F or so.