Has anyone tried the ThermalFeet foot warmers?
They fit over the toe of your boot and you place an air activated heat pack in them.
They claim that hand warmer sized heat packs will work too.
If you want to try something that really works, try Arcticshield's boot insulators. I've been using them for a couple of years and fell in love them. I don't know of any problems with the thermalfeet. Just wanted to provide another option that I know works.