The battery will often heat itself in the cold, so insulation is the main thing. My worst cold performance was when using a tight metal box to hold not so great lipos. So they would start out from the home ok, then sag like hell after a short ride in the snow chilled them enough.
Another time I had problems was when it was so cold I did not want to ride 15 miles to work in it. Around 15F. So I got the idea to bus the first 6 miles, then ride in 9. On the front of the bus rack, my lifep04 battery got very cold, and barely worked at all. Packing some additional insulation inside the battery box helped a lot in this case.
So the low hanging fruit here is obvious, start warmer if possible, and then just some insulation will help after that. Never had a bit of trouble with cold when using a padded triangle battery bag, it was insulated enough. That bag was a bit too insulated in hottest summer though, so I would ride with it half unzipped then, and try not to pull max amps.
Anything can be used to insulate a battery, including the crudest wrapping with packing foam and tape. A bike like the stealth could be covered with neoprene cut from an old wetsuit from a thrift store, or just cut a quilt to make a quickly attached coozie for the battery area. It need not be perfect, just needs to stay on when riding.
Starting out warm works best by brining the battery inside when its cold, but that requres a safe place to put it. Dont burn your house or garage down like I did. (garage battery fire) If you need to heat a battery, the ideal thing for it is a reptile heater. Little warm pad that goes under a reptiles aquarium cage. Its waterproofed. Second best, just a regular heating pad from the pharmacy.