I do something pretty similar to what ig_daddy and his family do, because I got the idea of using a power converter from them. But I travel int the car with Bud almost every weekend, (boyfriend 2.5 hours away, parents 4.5 hours away) so I've done a bit of investing in his comfort. The power converter was about $45, the extra-long heating pad about $18, the pet carrier was about $15 at a flea market, and about $15 for a big duffel bag to carry him out to the car in the winter. WHICH he is quickly outgrowing.
In the summer I just tell him to be good or else, and carry him out to the car on his leash. 
Before I bought all that, I used to do something a little different and less expensive. I used the duffel, and LOTS of those little handwarmer thingies. People actually ship their reptiles using those; they can last up to 8 hours.
So what I did was line the bottom of the bag with two hot water bottles, and put handwarmers underneath the hot water bottles. Then I put a layer of iguana, and then I USED to put a layer of blankets. I wouldn't recommend the blankets anymore though, because Bud would climb on top of the blanket and create too much insulation under him to be warm. I had the carrier back then, too, and I would just put more of the same inside there, so that he couldn't avoid being warm. Also used to crank the heat up quite high in the car; wouldn't recommend that either in terms of getting pulled over for erratic driving.
Before THAT, I used to transport him in a huge rubbermaid storage container with ventilation and heat like above. That was when he was a lot littler, and couldn't leap out every time I opened the lid. Then I lost the lid. Also I couldn't stand the scrabbling noise: he was constantly scrabbling on the side of the rubbermaid because he couldn't see.
Where are you going to get handwarmers in the winter? Online hunting and camping stores, for one, but I find them year round on the impulse buy shelf in the checkout line at Walmart. I don't even think the Walmart people know they're still there, but you could try your luck.
My number one tip is to travel at night so the little bugger is fast asleep and trouble-free. That way, he'll tolerate lower temps (nothing's gonna be as hot as his heat lamps) and not try to escape your efforts to keep him warm and safe.
Anyway, sorry this is so long. Hope it helps.
Have fun!