We make our own fondant from simple ingredients: sugar, water, and a dash of vinegar.  Cooked, cooled, and cut into rectangles that are about 3/8 inch thick.  These can be placed in the hive as a winter food reserve.  4 or 5 lbs will create a layer that covers a 10-frame hive, about 3/8 inch thick, and with the use of a shim, more fondant can be fit into the space created.  During a winter thaw, take the cover off and without lingering long enough to let a lot of heat out, add more to any voids where the bees have eaten the fondant.