I am in the middle of an EV build of a RAV4, and am using 50 Thundersky 200AH batteries (160VDC nominal, 190VDC max). I have 2 battery racks (front and back). I am from Winnipeg, where it gets to -40C or colder, and I plan to drive this year-round.
I am also using insulated battery boxes - If you have the head-room then add insulation on the top (heat rises) - not sure if I have the room or not (once BMS is added etc...).
I plan to use soil heating cables (Gro-Quick) - I will snake/router a path into the bottom layer of insulation and insert these cables. They come in 2 types - internal or external thermostats - I have the ones with a thermostat built into the in-line cable (I will try to put this in the middle of each pack). The other type requires an external thermostat controller. They also come in regular and heavy duty versions, and many different lengths. I am using 2 of the 20ft versions from here:
The plan is to always have them plugged in to an external AC line in winter - once you are driving they should generate enough internal heat (plus they have thermal inertia and should not get cold too quickly). I may also have a timer so they are not consuming electricity all the time (I will experiment) although they are only 100Watts each...
My feeling is the battery temperature makes a huge difference - keeping them warm will improve performance and will lengthen their lifetime...
Hope this helps!