We have used those heating pads designed for gardeners to keep seedlings and germinating seeds warm. That works, so I imagine something wrapped around the carboy would be even better.
But unless you're way out of temperature range, I wouldn't sweat it. We have beer fermenting all over the place. Outside, inside, wherever we can find room. If it's cold it slows, if it's hot it goes faster. I'm not saying that there isn't an ideal, but it works just fine in a wide temp range. So unless you're really particular or have a very cold fermenting environment, it's probably not necessary to heat the fermentor.