I have a few more suggestions. I do dog sports as well as camp for dog sports. So the whole point of it is to take my dog somewhere. Hence more practice than your average human at taking dogs places.
1:
Do not discount the power of those dorky looking silver reflective thingies! In your windshield! In the back over your kennel top! I used some Refletix insulation from The Big Blue store all though most the big hardware stores have it
https://www.lowes.com/pd/Reflectix-50-sq-ft-Reflective-Roll-Insulation-24-in-W-x-25-ft-L/3011904 to make custom fit ones for my side and rear windows.
2:
Learn about and come to love the ventlock! especially if you drive an SUV or minivan.. this tailgate lock is the best thing ever!
http://www.ventlock.com/ in the US you can get them from
http://www.cleanrun.com and
http://www.leerburg.com (and I think Leerburg from Amazon as well) They let you pop your back hatch up some number of inches while keeping the car locked. I own a 6" and a 12" and I use the 6" ALL THE TIME. It is in my car holding the hatch open in the underground parking garage right now, while I'm at work. Also anybody with a modicum of welding skill could make their own really it's not that complicated. (I also used it in the trunk of my VW Sedan with the seats down and the dog kennels on the seats)
3:
Experiment without your dogs. Get yourself a thermometer, put it in the car where you keep your dogs. I use a small digital indoor/outdoor one that records the daily high and low temps (over a 24 hour period) as well as displaying the current temp
https://www.fleetfarm.com/detail/acurite-suction-cup-thermometer-white/0000000095135. I check it (pop the hatch and look) most every time I return to my car, it takes a second and I have a REALLY good feel for what the temp of my car does, regardless of if it is 95F out or -40F out. (Both of which I've seen in the last 6 months, got to love Minnesota)
So I drive a dark colored SUV, and with these things I can keep the internal temp of my car roughly equal to the outdoor temp. When I drove a white car I could keep the interior the shade temp of the outdoors. The important part is to understand how the air flows through your vehicle.
Best results come with a vent at the top/front of the car.. so I'll vent the front windows an inch or two (I also used to use my sunroof but I don't have one in this vehicle) I'll put the shade covers on the windshield, over the kennels in the hatch and on the sunny window side of the car. (and if it's noonish both sides). This makes my car basically dark inside. I do try to park in the shade as well.
With the hatch popped and the cooler air coming in over the dogs noses/open kennel fronts, the thermometer I keep there seldom goes up more than a degree or two over the outside temp over the course of 30-45 minutes. And I've experimented with all day without dogs in the car and it still stays pretty good only a couple degrees F over the daily high temp.
That said, I do a lot of experimentation, limit the time they're locked up without supervision, and check my thermometer all the time even when they're not in the car so I know what to expect for performance in all conditions. It can be done. And sure it takes me an extra 3-4 minutes to get all the venting and shade covers in place when I park my car, but the peace of mind of knowing my car is locked up and my dogs are good for the time it takes to go to the facilities and grab a snack, or even a quick lunch, makes it worth it.