How can I hang a piece of blackout material at the window to keep the heat out?

[Sunshineandrainbow]

I am trying to keep the heat out of DD room. Yesterday I found an old piece of black out material, I used velcro to stick it to the top of wooden blind but it keeps falling down. Any others ideas, I am hot and bothered and can't think for myself!!!

Hang something outside the window to stop the heat hitting the glass

Sticky tape or blu-tak the top to the edge of the frame so that it's held firmly when the window is closed, same on the lower bit if it doesn't overlap the entire height of the window.

If it's longer by about 6 inches, you can just trap it outside by closing the window over it.