This is the best idea I think. However, you could just use the cooler. I would wrap the warmers and put them on one side of a small box, but big enough box for the tortoise to get off the heater if he wanted and maybe some substrate of paper towels on the other side.I have these too. My thought was to put my tortoise in a small box, inside a cooler with the hot hands in it. If that makes sense.
I will argue though, that Hot Hands are cheaper and more readily available for the general population, so as long as you are smart in your usage of them with your tortoises they would work perfectly fine in any kind of emergency.
When you buy a ten pack, shipping heat packs are $1-$2 each. You need something cheaper than that to ensure the safety of your animals? We have shipped hundreds of thousands of animals through ShipYourReptiles. I can tell you instantly the number one cause of DOAs- misuse of heat packs, overheating and causing the death of the animals, including turtles and tortoises. Breaking down that number one cause further, the most common denominator issue? Using Hot Hands/hand warmers instead of a shipping heat pack. It is incredibly sad and discouraging to see people still recommend their use.
I wouldn't use a hand warmer within ten feet of a live reptile. I know very well the damage and DOAs they cause. Paying $2 for a proper heat pack seems incredibly reasonable and smart. Taking a short cut for an emergency situation does not.