To meet the "perfectly coaxial" part I would probably do mostly what French suggested, with the addition that I would face the two ends of the rods so they were nice and perpendicular to the axis. Then mill two flats half the diameter of the rod (+ a thou or so for clearance), drill and tap one, drill and countersink the other (if the diameter allows) for a cap head bolt, then radius the ends so it rotates. If you only radius one side, it will index on the machined faces and give you a solid "perfectly coaxial" reference. Tighten or loosen bolt for tension, or maybe throw a small Bellevue Washer in between.