Use the M8 threaded rod and two nuts on the ends. Supports for nuts are parameterized - modify if you feel it necessary.
UPDATE 19-09-2012: Parts slightly modified. Idler end fits F608 bearing. No bottom endstop - belt spreader instead.
UPDATE 23-09-2012: Modified idler end.
UPDATE 27-09-2012: Modified carrige.
After printing, remove the support cylinders covering the holes for the rods.