I tried a heating pad at the last 3 races.
On the lowest setting my lipo's got to 109*. I'm going to try an old soldering station control on the heating pad, all I want is 80 or 90*. If you don't have a temp gun and don't watch the temp very closely the first time don't try it.
On the way to the track I put my lipo's in my inside pocket and they were 85* when I got to the track. It was 11* outside and 55* at the track.
I was cold as hell all day but my lipo's were just fine