I've used a belt on carboy's with no problem. But I prefer a heating pad with a three setting control so thatI have a little adjustment. I make a sleeve by wrapping a heavy towel around the carboy and slip the heating pad in between the towel and the glass.
BTW do you do your white kits in winter so that you can keep fermentation temp down. I do my whites at 60 (start at 65 and drop to 60 once the yeast kicks in)