I had my first experience with this when the power when out riiight after I got six new snakes. First time dealing with this all and there was a ridiculously long delay on my 40HR heatpacks... so I had nothing.
BUT! If ever without the heatpacks, all you need to do is use a very, very good ziploc and fill it with hot water and pack the snakes as if to ship. You have to make sure there's a good barrier between the snake and the water though, because it's HOT. My snakes were cozy the whole day. :]
I didn't have anything for my two milk's but I nabbed a small icechest which heated right up and worked like a charm, but for the majority of the time I had the ziplocs of heated water resting over the hides where the snakes were hunkered down and it kept them nice and warm as well.