Well, I had a look and I've come up with an insulation company that might produce the best acoustic treatment material around.
I had a look at the specs on the rigid panels and the co-efficient is 0.33 at 125Hz for a 2 inch panel. This is better than Roxul or Owens Corning for their 2 inch panels
I would imagine that it isn't cheap but it would do a superb job. I'm still going to ask them tomorrow for the pricing if I can get them on the phone. I will report on my findings
Ben