A folded piece of translucent white plastic with a hole cut in it diffuses the on board flash nicely. I was lighting a match and taking photos with this setup.
A plastic material used in the sign industry for temporary outdoor signs. This one is about 3mm thick with a hole for my lens. It was a little too thick for good light penetration from the built-in flash but might be good for a powerful external flash.
This was my favourite setup for a long time (I later added a light guide, see the next photo). This is the excellent Tamron 90mm lens on a full set of Kenko extension tubes.
I used this a lot and it's responsible for the sample photos. it's a phone book cover folded and lined with foil that sits over the flash to guide the light. Adding the phone book gave me at least an extra stop of light. The front piece is my favourite translucent plastic diffuser material.
A cardboard tube fitted over the built-in flash with diffusion material on the end. This was not the best build - I think the tube was too long.
I used a full set of Kenko tubes and the cheap Canon 50mm lens.
From the front. A cardboard tube with diffusion material.