There is no real solution for the problem you are mentioning. -15 is not what I call "cold" anyway. If you want to warm up your camera body during the shoot you take a high risk. The residual humidity in your camera can condensate and froze, making more damages than you expect. You really need to avoid thermal shocks as much as you can. Cold make your material more fragile, contracting the alloy of the body, harden the sealing and so on ...
Either you shoot from a controlled environment, like a cabin, or either you seek a better material for your shoots. Medium Format digital backs are made for this, especially the "+" line of Phase One. Try to avoid as much as you can electronics controls like AF and go manual lenses. In the manual lenses there is different behaviours between metallic lenses and composite lenses. When you will face -40 and + temperatures ... you will develop a more scientific approach of your material
When you plan to go in "very cold" zones, you can train your setup in frozen warehouses. If you have one of them around you, like meat or vegetables warehouse you can test it under - 25°C.
Try Nikon pro bodys like D4