I looked high and low through these forums to find a black material that wasn't GOM to use around my Seymour XD screen for my false wall. I don't recall the product, but it was found at a local fabric store. It is super thin and airy. Looks like velvet of sorts on the outside, but I wasn't fully convinced until I had a local guy with some nice AV monitoring and calibration equipment do a favor and just measure the difference of sound with a full range speaker with and without the fabric in front of it. His equipment is pretty high quality, and it DID detect differences, but very minimally. He was impressed that it did so well for a cheaper material. I didn' think it was THAT cheap until I used a Jo Ann Fabrics 50% off coupon, then it got "cheap".
I really wish I remember what the stuff was. It has some stretch to it, but not like spandex (which wouldn't be as transparent either). All I had was the "blow test" where you see how easily air blows through it. It was easier to blow through than the Seymour screen.
I did my own A/B test and made up a quick panel of the stuff that was just larger than one of my tower speakers. Without looking (I just covered my eyes) I tried to hear differences using both test tones, dialog, music, and movie samples. I could not hear a noticeable difference. She was annoyed with my testing after about 15 minutes.
Just tossing that out there that there are other products, BUT keep in mind that they aren't always flame resistant, etc. That is important too. Not sure how anything will ever start on fire with my screen wall. I guess that the amp in one of the subs could catch fire some day, but otherwise there are no lights, and just audio signal over audio wires near it. The wall is a couple of feet from any electrical outlet, etc.
Your setups may be different and then I would definately look at something flame resistant.