I first made one of these around 1980 and I am unable to recall the circumstance of my inspiration. It's quite easy to make and the results can be quite good depending on the care you put into it and the materials used.
This demo uses household aluminum foil which has the drawback of being unpolished resulting in some difusion of the sunlight.
I once made one with the blank side of an aluminized mylar balloon and it proved to be a very good reflctor. I remember that I was able to reflect a near perfect inverted image from the window onto the wall beside the window.
Caution is advised! Focused sunlight is VERY intense, and this is so lighweight, you can flash yourself while handling.