The safest way to move them would be a combination of the two ideas you suggested. Bag them and place them into a tub or tubs (depending on how many and what sizes you're talking about). Or even better, place them into an insulated shipping box. A styrofoam cooler could work well, too. If they're well-insulated, they won't even feel the temperature shift from house to car. Pack them securely so that they can't shift around (ie: can't get bounced around if the car comes to a fast-abrupt halt). Have the car warmed up ahead of time...that's a good idea, but DON'T put them on the floor of the car near a heater vent. That space can get way too hot. If the car itself is warmed up to where it's comfortable for you, the snakes will be totally fine, especially for such a short trip.