Best Sound Absorbing Material for DIY Wall Panels

I recently built a new house with a dedicated home theater. One of my projects is to completely cover the back wall of the theater (behind primary seating area) with fabric to help with the echo that I am currently experiencing. I have already purchased the fabric (16 yds)and plan on using 1" pine to make the panel frames & supports.

My problem is that I can't find an insulation material to fill my frames with before I apply the fabric. I am looking for an insulation that is semi-rigid so that the fabric will have a uniform (flat)look. It also needs to be 3/4" thick and preferably be sold in 45-55" wide rolls. Oh, I also want it to be relatively inexpensive (<$150 for approx. 168 sq. ft.)

Am I asking too much? :)

I have found some information on the Owens Corning website. It looks like they have what I need, but their customer service doesn't know where I can buy it. She put me in touch with one of their local reps, but he hasn't returned my call.

I have also found a material called Hushcloth, but it appears to be fairly expensive. ($2-$3 per sq. ft.)

Any suggestions on how to kill the echo without emptying my pocket?

You get bonus points if your answer involves me using the fabric that I have already purchased!