Z Stabilizer Top-of-Rod Clamps (8 or 12mm rod)

After upgrading by Printrbot LC with longer (400mm) 8mm smooth rods and M5 threaded rod leadscrews, and installing z-axis stabilizers for the bottom of the rod (http://www.thingiverse.com/thing:46082), I still wanted more stiffness.

Other clamps on Thingiverse were not to my satisfaction. Some coupled the smooth rods together by clamping to a rod (or rods) that were not in the same vertical plane as the ones used for motion, creating unnecessary stress at the points where the rods are fasten