Here’s a super-simple set-up for transferring analog film to digital:
A lens (or a macro lens, or a lens with extension rings) is inserted into the circular hole in the front. A negative holder made of two sheets of vinyl with spacers is at the opposite end. After the 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 an LCD display.