If your belt is warm to the touch then it is ample for warming wine. about 72 F feels neutral to most people. so if it is warm it is higher than that, covering the carboy and belt with a towel or something to insolate it decreases radiational cooling, once the container which will have it's own heat transference rate is warmed equal to the belt the must will also follow suit. liquid conducts heat 25 Times faster than air. So at this point the must/wine will heat evenly and maintain a more consistent temperature.