Thinking about battery boxes and their heating. What I have in mind at the moment is having one or two battery boxes either in the engine compartment, in the trunk or both. I find myself visualizing the rear battery box in the trunk more than the one in front. Probably because the trunk is a nice flat area which will be easy to work on whereas in the front one has to carefully think about how to secure the battery box in place and also make it safe in the event of a crash.
In both cases I'm thinking of angle iron based chassis with possibly aluminum on the sides and 20mm of Finnfoam insulation laced with self regulating heat cable on the inside. The cable can be had in 10W/m variant ready to be plugged in so I guess the only thing to figure out is how much of it to put in. Probably some overkill is appropriate as always. The way this would work is that the cable would be connected to the same AC power bus inside the car as the charger so that when you plug in the car it would simultaneously start heating the battery boxes and charging.
If one would like to make it a little smarter I guess one could device a system which would not start the charge before the battery boxes reach a certain temperature and perhaps also automatically control the heat cables so that they would only turn on if needed. They might also get turned off when the charging starts. This way I could use a more powerful charger. If I used a 3kW charger I couldn't use the heating at the same time. Especially since I also have to leave some room for a cabin heater which can use from 650W to over a kilowatt of eletricity. The limitations come from 230 VAC sockets usually being behind a 16 A fuse which gives us about 3500 W to play with.
Perhaps it's just easier to go with a 2 kW charger and have the possibility of turning on all the heating and charging at the same time. At 116.8 volts that means a 16 A charging current which is not very much, but still enough to get the car fulle charged overnight or about 10 km of range per hour if topping up on the road. At 3 kW that would be about 15 km per hour.