The rig was made of standard aluminium U-Profile (forgive me if that is not the correct technical term in english). As a counterexample to Murphys Law I was able to find it with almost exactly the right size to accomodate two cameras.
After measuring the size of the cameras I came up with the following diagram:
Using a standard jigsaw I then ended up with the following rig:
To be able to attach the camera with a variable stereo-base, I needed slot-holes. Having only a drill and rasps this turned out to be a rather tedious job, but after 2 hours the result looked okay:
Finally using two standard camera screws, the rig was finished.