you only need to keep your dragon warm, not basking hot. As diamc mentions chemical heat packs are a good thing to keep on hand. You can also use hot water bottles (heating the water on a sterno-stove, camp stove, grill, campfire etc.
As for heat packs, this brand designed for shipping animals and there are different sizes and durations
http://www.amazon.com/UniHeat-Shipping-Warmer-hours-4pack/dp/B006YEX6MA/ref=sr_1_1?ie=UTF8&qid=1411695713&sr=8-1&keywords=Uniheat and do not get as hot as hand warmers (still not intended for direct contact). Both types need oxygen to function so I'm not sure if wrapping it up in a couple layers of towel might smother it or not - might need to leave part of the towel open or take the pack out and shake it up every now and then. I think the shipping packs are intended to be taped to the lid/side of a ventalated box, the pack should have instructions on it.
Last winter I read about folks making foam coolers (again, the packs use oxygen so ventilation is super important) to keep their snakes warm. The cooler helps hold the heat in and they would use hand warmers or hot water bottles for heat.
In a pinch you can also use your own body heat, just keep him against your chest under a blanket or something.