This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
Make a small table as wide as the camera is long and 3 in. longer than the camera is wide. Sink a screw nut in the center of the under side to en- gage the regular tripod screw. Fasten a double or two-way spirit level on the front left-hand corner. Nail strips on both ends and on the rear side, to form a shallow box with three sides. The illustration shows the construction quite plainly. This device was used by a correspondent of Camera Craft as follows: The table was fastened to the tripod and carefully leveled. The camera is placed at one side, bringing the back snugly into the corner on that side. Make the exposure, change the film, slide the camera over to the other side and make another exposure. The table being 3 in. longer than the camera is wide, the lens will be moved exactly 3 in. when the camera is moved over to the other side. Three inches is the separation of the lenses in stereoscopic cameras and the negatives made as above will be the same.