sounds like a bad idea that may melt battery cases. other issue is that without a box and insulation your batteries will radiate all that nice heat rapidly. try some heatline and a space blanket!
I know it is a lot of work but since heat rises, you should put the heat blanket under the batteries to get the best heating. then insulate the sides or the whole batterybox.Well, it did make a difference. I didn't have to use a lower gear on the big hill going to work. The rest of the day it is in the shop, so the lamp is not needed.
I am looking for a small heat blanket to use over them, but haven't found any.
No problem with heat damage as it is only a small watt lamp.
When I get the second pack inside the car, the oil heater will take care of them.
Just experimenting, without breaking the bank.