So I have have tried several things to black out the windows in the back of my TV. Card-board held in place by duct tape works great in the winter. But when Summer comes it's gets so hot it melts the glue on the tape and falls off.
Curtains are nice but they are extremely difficult to engineer in place. And they don't truly black out the windows.
I was walking in home depot when I spotted sheets of foam insulation. I started thinking. So I bought a piece and cut out window shapes. I painted the outside with metallic silver and the inside black. I used spot dabs of silicone glue to hold the foam to the windows. It blacks out the windows and it added a barrier of insulation. The inside of the van in noticeably cooler now. Just use a bit of duct tape around the edges to seal out the gaps so light can't get through. Since the tape is not holding any weight it doesn't peel away as easy.
