I have also been investigating ways to warm my batteries. Right now I have about 12 miles of range, and I need to jump that back up to 20 miles so it can be an all year EV. My batteries are usually around 50-55F on a good winter day with insulation alone, and they need to be over 80F for proper battery performance. I am going to try an electric blanket, because they are inexpensive and have temperature control built in. I only really need it in the mornings after charging has completed before I go for a drive. The insulation on the batteries can keep them warm/hot but they need something else to get them up to temperature.
Tomorrow I am going to check out some local stores to see what they have for electric blankets, and if I do fine some I will try them out this weekend. I have to redo my insulation anyways, so it should be simple to wrap a blanket around the batteries before I tape up the insulation.