that has no perfect solution.
On balance you'll find it's probably best to leave the machine in the boot of your car. Presumably you have a laptop bag, but if you don't, I advise you to get one. Buy one that has plenty of padding, and if you're worried about heat, wrap the whole thing in a car rug or beach towel.
The rug/towel will act as an insulator, and the computer will be fine.