+ 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.