Iíve seen occasional references to view cards or viewing frames. Hereís my vari-format viewer which I couldnít survive without. Made from nothing more than 1 inch or 25mm strip pine, wood glue for the 4 corners and duct tape for the hinges. My prototype, which I lost in Yellowstone 3 years ago, was made from foam core. I moved up market with this one!
Next post will have a couple of construction shots.