Hi Phil_Cox,
How long is your flight? Typically our launch-to-land is about an hour, as the balloons are designed to rise til they burst. A 9 volt alkaline battery is sufficient for that time span. We no longer use the Sparkfun heating pads. Nichrome wire on a kapton taped enclosure works well. If you have access to a freezer that goes really cold, you can get a handle on the current needed to keep your particular package at the proper temperature (it's about -40 at apogee.) An insulating styrofoam enclosure is also good for protecting the stuff when it crashes lands.