Here is a magnetic oil pan heater that in conjunction with the block heater would probably work well.
http://www.jcwhitney.com/OUR-BEST-UL-APPROVED-MAGNET-MOUNT-ENGINE-BLOCK-HEATER/GP_2000836_N_111+10201+600001648_10101.jcw
Is a battery warmer worth the money? I have done some of my own research and found that this company is a little overpriced.