I agree with MrVampire, that'd be a great way, but just be careful!!
I took a 5-hr trip with mine just a day ago.
I have a 1gal critter carrier, fille dit up about halfway with warmer-than-usual (but not too warm!) of water, had the bottom bare with only his thermometer so I could see the temp and some plastic plants floating at the top because he loves hiding <3 I kept him between my legs or in my lap the whole time (yeah- I got wet a few times DX) which kept him warm, and kept the car a bit warmer than normal too. I had a towel over top to help with splashing and keep the ambient temp out ^w^ His temp didnt drop below 75 (from 81!) in the entire 5 hours.
Idk, maybe that'll help. Good luck, happy tavels :3