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!
Go to a store the sells Ductboard (used for making ducts for HVAC applications) It is 3/4" compressed fiberglass in 4x10' sheets.Use extensively by the mfrs. of the commercial panels. My favorite wall treatment is a sheet of this cut to size and an oriental rug over top. Looks good works great!
Read the articles on Acoustic Treatments here:
Check out klone-audio for pictures of diy room treatments. This is a must see site for anyone that wants to build acoustical panels. "A picture is worth a thousand words". However I don't believe that just covering your back wall will eliminate your echo effect. I will soon be building some panels similar to the pictures that you see at klone-audio and plan to place them at different room locations to experiment with my room's acoustics. Good luck!
Owens Corning makes a two foot by four foot compressed yellow fiberglass.It comes in thicknesses of two to eight inches.Careful what you wrap it with so it absorbs and does not reflect and not serve the purpose.Go to Jon Risch's web pages and read,read,read then build.Remember:measure twice,cut once.
I did this in my last home theater and it worked great. If you have a table saw and have 2x4's cut them to fit a Owen's corning insulation fibergglass. I think was 12 inches wide. I got the Fiberglass with the paper backing, built the frames with wide side fitting the fiberglass, then stapled the paper backing to the back of the frames. I then purchased very attractive fabric at a high end fabric house and after pulling it tight around the frame and fiberglass, stapled that to the back of the frame. They sounded and looked so good the guy buying my house wanted them included in the purchase of the house! I used them sparingly as you don't want to damp too much. 8 I think. Wish I still had them, but have been building some diffraction DWAL's from blueprints I found at the Decware website. Man, do they work like magic. I wish I had more time to build about 20 of these things! Unfortunately, they use Thermafiber (which you can also use instead of the Owen's corning, but it doesn't come with paper backing) which absorbs more uniformly the frequencies than the pink OC and much more MDF than I expected. I love the DWAL's though. Next, think I'll try some SWAL's.
Thanks everyone. Great resources! I think the ductboard is exactly what I am looking for. I'll start with the back wall and experiment from there. I may tie in some curtains on the side walls if I am still having problems. This might help keep the wife acceptance factor high. She isn't thrilled about the fabric covered back wall.