New: now with template! foldable recycled mirror popup flash bouncer
Update: I added a pdf template you can print out to recreate this, even without the box (although you'll be missing out on the treat) if you line the thing yourself with some foil!
A commenter suggested using a milk carton, which at least where I live (and he) are tetrapak-like and lined with shiny silver foil.
A4 format: popup flash bouncer A4 pdf
Letter format: popup flash bouncer letter format pdf
I recently had some delectable snacks "Schaumküsse". They were in a nice cardboard box with a mirrored silver lining (haha!).
This gave me an idea - I had to use it as a lighting accessory! After a bit of cutting, scoring and folding, and a few paper clips, I came up with this foldable lightscoop-clone.
It bounces the popup flash onto the ceiling or, if the camera is held in portrait orientation, the wall. See next photo for the kinds of pictures it takes... I think it may be more efficient than the original lightscoop, since it has mirrored surfaces all around, not only at the front.
Original product: www.lightscoop.com/ (includes some instructions on use that also apply to the diy version)
Instructable I modified: www.instructables.com/id/Fifty-cent-flash-bounce/