Placing the heat pack inside of a towel, and that inside the cage, would be a good idea. It will help prolong the duration of use. I would not use more than one.
Keep in mind that in an emergency situation like this, your goal isn't to get the cage hot, or even have ideal temps, but rather to keep it from getting TOO cold. Most all reptile species will be fine for a day or two at just normal "room temperatures". I would be more focused on keeping a portion of the cage at least 60-65F, as opposed to trying to achieve a specific and usable hot spot of 84F, or 90F, or whatever your ideal husbandry goal is.
It is the same philosophy/strategy with shipping the animal overnight. Trying to keep the box warm or "hot" often leads to DOA. The goal is just to prevent the box from getting TOO cold. The animals can handle cool temps much better than they can handle unescapable hot temps.
Super smart to have a handful of heat packs on hand for such an incident, or for a vet trip, or car ride, etc.
And I favor the 72hr heat packs for this. They don't peak as high as the 40's, and last even longer.
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