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.
Step 1: Puncture the pie tin.
Step 2: Lay the foil out.
For this demonstration, I just wanted to get it done quick for photos.
I did take care to tear the foil carefully to avoid wrinkles.
Step 3: The adhesive.
It worked well enough, but I recommend epoxy, or some other adhesive that doesn't require evaporative curing.
Put it all the way around. If you use a 5 minute epoxy, you want to get it all on in a couple of minutes.
Alternatively, you could put invert the tin on the foil first, then epoxy the seam.