I've considered using 2 old reflective windshield shades as material sources for something like this. I would think this would be a HUGE benefit here in Central TX.
I thought about cannibalizing windshield shades I had, but a roll of Reflectix was 20-something bucks. And I've got plenty left to do the back glass on the hatch.
But yeah, this'll help so much in the summer. Mostly did it for that reason, but it'll add a little privacy when sleeping in the back as well.