Thanks for that FancyBits, I will check that out.
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You could try a search for siver screens.I haven't done so myself so I don't know what results you might get.But I have got them for my car & campervan.They certainly keep the heat out in the summer.:D
Space blanket. Good if you get stuck near Everest or for keeping the interior of your car cooler if your stuck on a roadway in the Sahara.
Bacofoil ?
I had a pair of solar panel fans which slotted in the back windows. They kept the temperature reasonable when we had the 2 Boxers.
Try car canopy. Good luck!
I think the search term you're looking for is "air-conditioned portable garage"