Providing heat during a winter move?

I have searched for and read all the posts I could find on transporting ball pythons when moving, but have not found detailed information on providing heat during the move. I will be moving with my three ball pythons from Delaware south along the east coast next weekend, so there is the potential for the trip to start out very cold and end up very warm. We'll be traveling maybe 14 hours and they will each be in their own snake bag and then placed in a plastic box. The box will be on the front seat of my truck and I'll have a towel available in case I need to cover the box due to bright sunshine. I will also have a thermometer in the box, but what should I use for supplemental heat and where in the box should I place it if it's needed? Thanks so much in advance!! ET, Sunny, Honey and I will greatly appreciate any advice that will help them arrive safe and warm to our final destination!!

hi. well we recently drove from el paso, tx to MA with 3 snakes and 3 rats and a car loaded down. for the snakes i just used small tubs with the UTH's taped on, plugged them into a thermostat and plugged the thermostat into and inverter that goes into your cigarette lighter. i taped all 3 tubs together, threw a blanket over them and put a seatbelt over that. worked out pretty well since i also kept the car warm too. that is what worked for me but i know other people have had other ideas that wroked well for them. here is a link to the inverters

http://www.amazon.com/s?ie=UTF8&rh=n...20power&page=1

when i did my winter move I used human heat pads and a power inverter, I stacked everyone in smaller tubs then put the smaller tubs into a large tub ran the heat mats down the center and down one side, use a couple thermometers to keep an eye on temps oh and kept the heat in the car higher than I like it. they all made it safe and healthy.

Thank you both for your responses, I appreciate it. Does anyone have experience with the disposable warmers that came in the box with my first snake? I'd love to be able to use those instead of investing in an invertor and heat tape I probably won't use again. I figured I'd be okay using them since we are driving straight through (no hotel stays), just no sure the configuration to use. Where in the tub (or outside the tub) I should put the warmers and how many to use. Thanks again!!