This is what I do when I want a quick diffuser for a pop-up flash. I use a 4ply piece of 100% cotton rag mat board cut to size and put it in the spot the flash itself collapses into when not in use, fits perfectly and stays there snugly.
Here it is in use. This works to allow SOME light to pass through the board (diffusion) and pushed a bunch of light up toward the cieling, acting as a bounce....
in a mirror, showing how it looks
in a mirror no diffusion