I used hand warmers, the ones you open and give a shake and they warm up. They do get quite hot so if you use something like that, put it inside 1 or 2 thick socks and tie a knot in the top so your bp's can't get in. You could even stick a probe in each if you have two digital thermometers. You can buy these at hardware stores. You can do a 'test run' to monitor how hot the heat bags get with the socks covering them before you go on your journey. Others may have better methods so keep checking back on this thread.