I would merely wrap the enclosures(if they are not large enclosures) with a couple blankets to insulate them, putting the heat packs on the outside or underneath.

Or if the enclosures are large, get smaller (just big enough for the snake and a water bowl)rubbermaid bins, wrap in blanket, put heat packs underneath the bin(with blanket underneath heatpack too). These would be probably double the size of a shoebox? perhaps larger? so that it's large enough for a little movement and for a water dish.

If the boxes are insulated, putting it near a fireplace won't help keep it warm much. I'd buy a lot of the 40hour heat packs to have on hand for the emergencies. You could store them in the rubermaid bins. LOL.