Keg King Heat Belt

The 30 watts will gently warm without creating a hot spot.
Heating belts are preferable to heating pads for the reasons that they can simply be moved up or down the fermenter to increase or decrease the heat transfer to the brew.
Heating belts are also preferable to heating pads because pads tend to heat up the yeast bed on the bottom of the fermenter which can produce some undesirable flavours in your beer.