I'm guessing you want absorption there instead of diffusion but that's a different topic. I have a sidewall of glass that I also need a solution for. I found this recently:
http://www.rosebrand.com/subcategory...-and-foam.aspx
I was thinking I could have someone make curtains out of this fabric for me. Have a look at the pictures, I guess this is the material auditoriums would use.
I was also thinking about a quick and dirty solution of just buying a few yards of that fabric and hanging it up when I want to have a serious listen.