I like to use 40 hour heat packs, just in case, but in spring and fall sometimes I use the 12 hour ones...they will work overnight, then cool when the day starts to get warm. When it's too cool both night and day, I want a heat pack that will keep working if there's a delay in the shipment.
I usually tape them to the styrofoam panel that fits on top, on one side, or to the inside of one of the side panels. I don't like to put them underneath, as I worry about too much heat on one spot overheating the snake--the heat packs can sometimes get over 100 F.
It's important to have a couple of ventilation holes when using them, as they do use up some oxygen. How many packs you use depends on the outside temperatures, and on how big the box is.
They aren't very expensive, so consider testing the boxes you use with heat packs and a high/low recording thermometer to see how hot they get inside when you leave them outdoors overnight, and determine how many you should be using.
Remember that if anyone READS the warnings you put on the box, it may not spend a very long time exposed to the low temperatures outside, so it should be able to have safe temperatures inside at room temp as well.
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