I've had shades on all windows for a while now and they for sure keep the inside cooler on a hot day.
I wanted them removable and also able to stay on with the doors open. I've found that the most reliable method is to buy grommets and install 2-4 one each shade, then use suction cups that can be secured thru the grommet. These are very reliable in having the shade stay secure. Don't use Velcro, or magnets, or cords.
We love ours for the cooling, UV protection, security, and privacy. And I keep the cargo windows shades on almost 100% of the time.