Here is something I made a few years ago - a 3D viewer with a bunch of stereo slides to go with it. I designed it to look like some kind of cartoony sacred ark.
It’s made from art board and coloured in with texta. The slide is mounted on the back, and viewed through the lenses at the front. The focus can be adjusted by sliding the mount back and forth with a tab under the viewer.
To view the nine 3D slides, you need to cross your eyes slightly until the two images merge into one. The resulting overlapping image will appear 3D. Some people find this easy, but loads of people will find it very hard to do. That’s why we make slide viewers!
The slides I made are a combination of 3D photography, and some manipulation of sketchbook drawings. I have also used some 3D photos of my cardboard dioramas, and cheated a bit in photoshop to make ‘em appear a little more 3D.