I put a small window AC unit in the right rear side window of my 4runner so traveling with my dog isnt a problem. I can leave her for short periods without worry. Ive posted here about it once or twice. I have a rear cargo carrier I built a cargo box on and have a small Honda generator in it to run the AC. I get about 5-6 hours of run time safely on a tank of gas on eco mode.
None of the many oft proposed methods I saw had much safe, real life promise to me. the boat and RV people figured this stuff out years ago, it works, its simple, pretty reliable. Id only add a temp monitor that messages you if the temp goes above some point, I dont have one yet but havent had to leave her very long at a time since about 2 summers ago. It works great just for around town stuff also. I start it before leaving the house and let it run on 62 degrees while im in and out of stores, the dog is quite chilled out, literally.