I bought the Thermacell heated insoles. They work good and keep my feet warm but I don't use them very often. Even if i take out my boot insole and put in the thermacell, it is still a very snug to tight fit with the thermacell. I wear a pair of Cabela's 2000g boots. I'm shure there are boots out there with more wiggle room that this wouldn't be a issue. My prefered method is taking double foil bubble that you use for the house and cutting that and put it in your boot. Keeps your feet warm!