Helix_2648 the easy way to make a simple diffuseur is to take a sheet of tracing paper A4 size to cut at the bottom a hole which size should be between 1 or 2cm larger than your lens cap. Trace this circle with his center at 2cm from the bottom of the sheet in the middle. Cut it and make two little holes at each extremes of the cut to place a rubber band. Now you've got a curved diffuser like the one you're looking for...
That's interesting MinitecaPhotographie , you could write a tutorial