Question for the colder weather bros

For my dudes who daily year round, I need some cold weather help. I keep the hogs in my unheated but insulated garage over night every night. However, all day long at work it sits out in the weather, which recently has been unusually cold. I’ve had to replace both batteries (02 fxdl in the summer, 15 flhr in the winter) just last week. But when it’s 9 degrees all day, she don’t wanna start at the end of the day.

It’s tight under seat, but does anybody have any way of insulating the battery? My charging system is good, I got that checked when my 1 year and 3 day old battery shit the bed. (Batteries plus, never again) The temps will be up this coming week but it’s only February!

HELP!