Heat isn't the only issue, the vibration and movement the car makes too. That rules out the vast majority of the ways we keep a computer cool. So your best bet if you buy a premade car setup or if you had to something with 0 moving parts, there a few ways you go about that but I let you decide how to tackle it. Also just because it's in standby doesn't mean it won't decide to wake itself up for some odd reason than inadvertently drain your battery. And if you decide to do this mounting a solar trickle charger to the battery may not be a bad idea especially with a safety cutoff so it doesn't overcharge it, it may be something worth looking into.
In my teens I wanted a setup like this where I had a computer in the car, now that I am an adult with a touch screen display lets just say it actually kind of sucks. It requires more attention to do something than the radios with buttons and tends to temp most people I've seen to play with it while driving because it's touch screen and lets be honest that's alot more fun than traffic.
A more preferable setup I found is a vent mount in the center for the phone to use as GPS or whatever your need is and set it up to the car via bluetooth or aux as for all intents and purposes it's a computer with an internet connection. Plus it's less tempting for thieves to break in to steal something that is not there or is just a stock OEM stereo. Plus a little bit of advice you can do little lunch dates with your significant other while their at work and eat in the car and watch an episode or two of your favorite shows with this setup, you can thank me latter as little stuff like this will make them happy and your life much easier.
I usually commute a couple hours daily, the more simplistic and stupid proof a system is the better so it won't distract you as much. You can be the best driver in the world but that won't stop the idiot trying to hit you.
Oh and from personal experience if you decide to go this route look into Anker for phone chargers and cables. Had various types of their charging equipment that is used and abused extensively for about a year now and is holding it great where as I would usually go though multiple sets of charging equipment by now.