For a small snake, I would use a deli cup with some sort of cushioning bedding, with air holes, with the rim of the cup taped. If you can tuck that into your bra, or an inner pocket next to your body, that would be fine. Otherwise, you'll need an insulated cooler or box, with a heat pack in a SEPARATE deli cup, and put both delis in the cooler. You have to be EXTREMELY careful to never let a heat pack contact the snake's deli cup directly. And really, 40 hour heat packs from Superior Shipping are safest. The kind you buy in sporting goods stores, for humans, get way too hot.
While you can plan for the car to be warmed up, etc., you also have to plan for what happens if you have a break-down, and get stranded. This happened to me on the way home from a reptile show, in the summer, when they closed down the Florida Turnpike for several hours to life flight a person out, and it was not possible to go forward or turn back, so there I was, in a car with a poorly-functioning radiator (so I couldn't leave it running, without moving) in 95 heat, no shade. Luckily I had a cooler with ice and was able to keep my newly-purchased gecko cool- but if I hadn't had that it could have been a disaster.
There is a product you can buy from Josh's Frogs, called a Phase Change Material Panel. (The packs leak- don't bother.) It's a plastic square filled with a chemical which stabilizes at 70F. (22C.) They are called Phase 22. So you heat the panel to 90F before going out, and it changes its temp back to 70 and stays there. In the summer, you freeze it, and it warms up to 70, and stays there. This product is revolutionary for shipping amphibians and reptiles. It was developed for shipping human tissue. One of those would work great for you, and you could keep it and reuse it for whatever temp you needed- warm or cool.