Is the towel supposed to act as a "blanket" of some sort? If so, it's useless anyhow. Blankets are good for creatures that produce their own body heat. Snakes don't, so they can't warm themselves under a towel or blanket.

The idea of putting hot water into plastic bottles (2 liter soda bottles or something similar) is a good idea. Do you have any human heating pads around the house? Those can work, but if they have an auto-shutoff, you'll have to keep turning it back on.

Can you make a run to WalMart or anything? If so, you can pick up heating pads (trying to find those that don't turn themselves off) and you can pick up hand warmers from the hunting or sports departments. Those can help, too.

Any space heaters you can use to warm the general area the cages are in?

Once you get some sort of heat source, wrap the cages in towels or blankets to hold in what little heat you're able to produce.

The cold is not an automatic death sentence for your snakes...and not even a guarantee that they'll get sick or anything. It just increases the risks of illness. They may get through this just fine...so try not to panic. Use your head and figure out what sources of heat you have around the house.