Instead of using disposable hand warmers, try these. www.reheater.com
The pack stays liquid and inert until you press on an aluminum oxide disk which triggers the reaction causing the pack to warm up (quite quickly actually) staying warm (130F this site claims) for two to four hours i have seen. To recharge the pack just slip it in boiling water for 20 minutes or so. They last forever and they are getting cheaper and cheaper. I got one over 2 years ago from the Outdoor Adventure Show in Toronto when they first came out.
