Three things moved the needle:

• Water temperature: Started heating water to 205°F instead of 200°F. In cold air, that extra 5°F compensates for the heat loss between kettle and brewer. If your cup tastes sour, go hotter; if it tastes bitter, back off.
• Grind size: Adjusted one notch finer than my usual home setting. Cold coffee beans are denser and can act coarser. If your brew runs too fast and tastes weak, grind finer. If it stalls and tastes bitter, go coarser.
• The towel wrap: Wrapped the brewer in a dry towel during the bloom (first 30 seconds). This held enough heat to keep the slurry at a good temperature. It's not pretty, but it works. A carafe cozy would do the same job.

Also, I stopped eyeballing the ratio. A small travel scale from RAMI's kit is worth its weight. I used 20 grams of coffee to 300 ml of water (1:15). Without a scale, you're guessing, and in cold weather, guessing gets you a bad cup.