I'd try one of the mounts for the Benelli M1 Super 90s, as many of the aftermarket parts interchange with the Beretta 1201. My only concern is that the radius at the top of the 1201's receiver is slightly larger. For instance, I had to file the upper contact surface on my Sidesaddle in order to get the remainder of the unit to fit flush against the receiver.
Also, don't screw the the retaining bolt for the mount (or Sidesaddle) too tight. You can compress the sides of the alloy receiver to the point where it will not allow the bolt to cycle.