Be carefull with adding a heatsource to your camera.
It can create moisture and get inside the camera,
especially if you keep it covered. When you cover
the camera, it retains the moisture as well as the
heat. I had the same problem with my 1 hour batteries,
so I bought the long life batteries.
They are a little bigger, but I can use the
same battery on consecutive hunts without
having to recharge them.