Some years ago I developed with some other friends a fold-able 3D viewer, similar to the Viewmaster from the 80's. I got some of the remaining lenses from that product and try to develop a newer version, but in this case using a smartphone instead of printed stereograms.
This time I used a plastic cardboard instead of the cardboard or construction paper.
I redesigned the whole frame to hold a smartphone Samsung S3 form factor. It also holds perfectly a Nexus 5.Earlier prototype
Earlier prototype
Latest prototype without the band strap