RAM Part RAM-B-108B-A-366U : Fits 1" to 2.1" diameter.  Standard 1/4" - 20 threaded post.  Angle is adjustable at the tip and base.  Vibration dampening technology.  Comes with the short 1.75" arm.  To mount the base, place the saddle shaped plate on the rail then wrap the metal clamp strap around the rail and plate. Tighten the screw knob on the metal strap with a screw driver until the strap has a firm hold on the rail.