Gareth @ MAKE found this simple setup for transferring your old film to digital. From the Flickr user svofski:

A lens (or a macro lens, or a lens with extension rings) is inserted in the circular hole in front. A negative holder made of two sheets of vinyl with spacers is at the opposite end. After film is inserted, the box is closed and camera is aligned.

Since any remote source of light will be hopelessly out of focus in such setup, no special diffuser is really necessary. I normally use LCD display.