One of the telltale characteristics of the Holga/Diana/Plastic camera is the vignetted edges. Cheap plastic lenses don't make for perfect edge to edge illumination on the film plane, and the fact that the negative is 6 cm x 6 cm means that the image the lens creates really has to be significantly larger than a 35mm film plane. To accentuate this effect, I remove the included matte frames from my Holga and Diana. These plastic squares crop the image area ever so slightly, which gives the final photo a nice clean edge on the negative, though the lens vignetting is still usually visable. Removing them allows the light to pass over more of the film, and creates a somewhat more rounded exposure. The falloff of light at the edges is no longer cropped short, and the vignette is more pronounced. One thing I also do is apply two strips of black electrical tape to the two exposed plastic joints inside the camera where the matte frames would sit. Because the film slips along these sharpish edges, it could stratch without the tape there.
-llg