The simple joys of stereo photography...

Here are a couple of the attempts at creating stereo (cross-view) images:

Nothing. Except you need to be able to cross your eyes. Here's a tutorial:
www.kula3d.com/how-to-use-the-cross-eyed-method

It can take some time to get a feeling for it. For me personally it helps to put a single finger in the middle (between the two images) not too far away from my nose, look at it and then vary the distance.

Something I'm not going to do:
"Eat the mushroom, open your mind."

I wish someone would make something like a Nishika N8000 adapter for medium format cameras. I know Lumix made a "3D" lens, but from what I recall, it was not very good.

Did you make your own device to do this?

You just need to take two shots from a slightly different perspective! No device needed. There are some ways (with a prism) do create one though… Has been mentioned by @3DGunner in another thread here.

There used to be rails which mounted on a tripod to which the camera was mounted. An image was taken and then the camera was slid along the rail for a few centimetres and a second exposure was made. The two constituted a stereo pair. There were special double slide mounts available which could be used in a binocular viewer to see the stereo effect. The resulting ‘3-D’ image was realistic if the distance between images was about the separation between the human eyes: it became more exaggerated as the distance was increased. I can’t remember when this equipment disappeared - it may have been the early 70’s. This is something I read about but could never be bothered to try…

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I‘m not sure what you mean… for viewing or taking shots?

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Actually, the 1/30 rule is a good start that has been used for decades by stereographers. It states that the interaxial separation should only be 1/30th of the distance from your camera to the closest subject.

For macro you need an inter-axial distance of the optical system much smaller than the interpupillary distance, and for distant objects it will be much larger.

This is a good solution: 3Dstereo Stereo Wide-View Viewer for Prints and Monitors

The bigger the prints or the screens, the more spectacular the stereograms are, if they are done correctly.

I must have been about 7 years old when I stumbled onto the fact that if you are in a bathroom with small-tile floors, and forgot to bring reading material, you can entertain yourself by crossing your eyes so that the tiles shifted by one tile relative to each other. The slight differences in the grout and the edge of the tile caused the tiles and the grout to separate, apparent-distance-wise. The same principle works with these SBS (side-by-side) stereoscopic images. Cross your eyes until you have a three-panel triptych, align and hold it steady, and the center panel should turn 3D for you. I've been doing controlled eye-crossing for 56 years, so it is easy for me, but you should be able to do it with some practice.