The photos below do not show all the mounts I have built but you get the general idea. For example, several mounts are just pieces of drilled out steel or aluminum mounted to a tripod adapter.
One problem with putting cameras side by side on a length of aluminum is that you need to put a hole and fastening nut for the tripod mount, in between the cameras, which means 3/4 to 1 inch extra gap between the cameras, spacing the lens wider than necessary.
That led to this bracket which puts the cameras on the top level and the tripod mount on the bottom level. These metal brackets are steel straps sold at a local hardware store, then drilled out for the desired spacing.
1/4-20 screws attached to knobs (available at a hardware store) are used to connect cameras on top of the top rail. I also have a version of this that has 3 levels – the middle level, like above, holds the cameras. A third level above the cameras holds mounts for shot gun mics. I’ll have to post a photo of that some day!
A seldom used mount is this very wide angle setup, which is made from two oversized calipers, used in drafting. The two camera mounts are adjustable to provide lens spacing of up to about 2 feet. But keep in mind there are very few situations (big landscape shots mostly) where you will ever need something this big.
I recently purchased this milled piece of aluminum bar which provides an adjustable camera mount. The center contains both a 1/4-20 and a 3/8 tripod threaded mounting hole.
After this photo was taken, I modified the bar slightly so I could mount it in the earlier 3D mount, above. With the adjustable rail, I can get the two Canon HF M301 video cameras down to a 2 3/4″ lens separation (center to center), plus where needed, I can also spread the cameras apart.
The rough rule of thumb is that the main object in 3D space should be at least 30x the lens spacing away from the cameras. At 6″ camera spacing, the minimum subject distance should be 15 feet (6″ x 30 = 15 feet). At 2 3/4″ we can cut that distance in half. And that is really a minimum – I find better results when the subject distance is kept beyond the minimum.
But there are other considerations when setting the camera spacing. For some situations moving the lenses further apart helps to emphasize the depth effect. For example, when I was shooting Hoopfest at 3″ spacing, the depth effect disappeared around 150 feet (or so) from the camera. But when I set the lens spacing to about 6″, the depth effect was nicely visible down the whole city block.