Horsefly
Solar Wizard
That will probably work.I will try to place heating pad on a L shaped aluminium sheet (kapton insulated towards cells), this way it is impossible to get any hotspots under the cells. Also with the temp sensor in between heating pad and cells, you can control precisely how hot the aluminium can get.
I know most people will instinctively know this, but it is worth saying: The pack of batteries has a pretty high thermal mass. That means that it takes quite a while to warm up, and once it's warm it takes quite a while to cool down. In the graph that I posted earlier in this thread you can see that the temperature at the top of the cells lagged 5-10°F behind the temp at the bottom of the cells (my heating pads and aluminum were beneath the cells). This means you may need to do some experimenting to see if the cells really get warm enough before you turn off your heat. It also means that if you heat too quickly the cells area furthest from the heat could still be too cold when the area nearest the heat could get dangerously warm.