I've had good luck with the old Pentax stereo adapter set. One piece (two front-surface mirrors) screws into the filter thread of a 50 mm lens (35 mm camera) and gives two portrait images on a 24x36 mm landscape slide with a blurred border between them. The other piece (two eyepieces) holds the 2-inch slide (conventional mount). With a proper foreground cue, the stereo effect is good.