+ It is very easy and quick to make. It takes less than 5 minutes to make it.
+ The mounting/unmounting is very quick
+ When unmounted it fits your pocket.
+ You will need only white cardboard, foil (e.g. from chocolate), and adhesive tape.
Scissors, pencil. For more accurate result cutting knife is also desirable.
Cut a rectangle of cardboard which has about 12cm width and about 15cm height. Exact dimensions are not critical.
Then make notches so, that your cardboard can tight hold on the ledge.
Better use cutting-out knife. Scissors are also ok, but not so accurate.
Place a cardboard on the ledge and see if the hole is ok. If not, expand it.
Take a chocolate foil and fold it twice. Cut the side to make its width equal with the width of the cardboard.
Place the foil below the window. Fix its bottom part with adhesive tape, wrapping it around cardboard below the window.
Congratulations! Your new bouncer is ready !
Place it on the ledge and bend the foil to form a parabolic profile.
Now direct flash is not illuminating an object you are shooting. Instead of it larger cardboard is glowing, giving much softer light. Moreover, the light, which passed cardboard will bounce from the ceiling, giving you ambient illumination.
You can adjust configuration ( foil form, foil size and cardboard size) during your shooting.
Go to the last step to see my samples and final thoughts.
I shoot in M-mode with ISO 400 - 800, and shutter speed above 1/100.
I also used positive flash power correction to make my photos brighter.
If you like the result you can easily make such a bouncer from nice durable materials. You can enforce the mounting part of the bouncer to make it fit more firmly.
You can also invent a way to attach such bouncer not only to SLR, but to some other camera having a flash.
Thank you for your attention :)
Update: See nice modification for close-up photos and comparision in this comment from chibimaddy