This is the first step in building the rig. I took a piece of 3/4" steel bar. I cut it about 8" long, so there was about an inch hanging over each end of the camera.

The flex brackets have 3/8" threaded sockets on one end to attach to a tripod or light-stand, and a 1/4" threaded stud on the other end for attaching to a flash shoe.

I drilled holes in the ends of the bar sized for 3/8" threaded holes, and a hole sized for a 1/4 threaded hole for the camera's tripod mount. This lets me use a thumbscrew to attach the camera, and since the hole is threaded the thumbscrew is "captive" and doesn't fall out. Thanks and credit to Jim Board (jb_va2001 on dpreview) for this idea.

I also added another 1/4" threaded hole in the bar placed to hold the quick-release plate for my tripod. That way I could attach the whole rig to a tripod.

I used a tap and die set to thread the holes in the bar.