Also take a look at Nordenmark mtbo mapholders: http://nordenmark.com/
Their clamping setup will fit a non-round bar but it's not obvious if the screws are far enough apart to span the one you mentioned. If it doesn't it should be pretty easy to modify. If the stock clamp doesn't fit you might even be able to get the owner Sven Nordenmark to make you either a custom clamp or an adapter to fit the stock clamp. He's a pretty helpful guy in my experience.