I haven't read the entire thread, but I just received the HeatShield roof sun shade. It's in 2 pieces. It's the semi-rigid roll-up shade, silver on one side, white on the other. It uses strategically placed suction cups, 6 per panel.
I have had it installed for 5 days, in and out of garage, 102f heat full sun for several hours, over 250 miles. They haven't budged and there is NO SAG. Silver side up, white inside...while driving, I don't even know it's there, and it probably reflects heat better than any fabric. I have pressed on the suction cups several times, and they don't seem to be losing their seal/grip.
Currently $44.95 for a two-piece roof sun shade, direct from HeatShield.