Bring your own shade - The solution
Success at last!
It took me a lot of years and sidetracks to finally come up with the easy solution to my shade problem. My dogs have been patient with me. watching as I tried one thing after another to keep them cool as they waited for me in the car.
Since I started this project I've replaced the van I started with. The car I bought in 2006 was smaller but no easier to shade. I tried putting in window screens like I'd had in the van but it didn't work like I'd hoped. I've been putting a throw blanket over the windshield and hanging towels over the windows. Ugly and only barely effective.
My new solution isn't very pretty, but it's less ugly than the previous one, and it does a very good job of keeping the dogs cool. Plus it's easy to put on and off. I can shade either side of the car if I want to have a side open, or I can shade both sides. Having both sides shaded sometimes blocks the wind a little, but I can minimise that.
All I needed was two cheap blankets. I bought twin size for my small car. A larger car might require larger blankets. I put them lengthwise across the car, with the side of the front one completely covering the windshield and held on by closing the doors on it. It's long enough to hang down and completely cover both side windows but I can fold it so the edge that's held by the door is doubled up and doesn't hang so low if I want a better breeze to come through.
The back blanket has a small hole cut in it so my radio antenna can stick out. It's held by the doors just like the front blanket, and it hangs nicely over the back window. Different cars will need different methods of holding down the back blanket, I suppose. For my purpose it seems to stay nicely without being held in back.