Build your own 3D dual camera mounting rails
Ritwika Mitra shows how to build your own dual camera mount featuring an adjustable interaxial distance for 3d stereoscopic photography or video. Ritwika is a high school senior, by the way. Cool young woman! She and her sister are also the founders of the non-profit Rennow.org.
The details begin at about 2:30 into the video. I built this but I also use a fixed mounting bracket that is simpler and smaller. At some point, I will post photos of my set up for 3D.