I know that some of us have DIYed solutions for covering the rear window of the GFC and there is at least one aftermarket product that I’m aware of:
I use Heat Shield window covers for the cab so I ordered another windshield one with the intent of cutting it to size. It turns out they can make custom sizes, so I requested one for the rear of the GFC.
I like it because it uses suction cups to attach to the window rather than having to install the magnetic tabs for other solutions.
Anyway, just sharing it here in case anyone is interested