Hey, glad that you are getting things figured out acoustically.
My panels that I made were using custom printed fabric so that they were the standard "solid colored rectangle." A number of people over at AVS use this in a 24" x 36" panel that simulates a movie poster, but I opted for some custom artwork to span my panels.
The company is called Spoonflower.com and the two best products are Performance Knit (my personal favorite for acoustical purposes and great print quality) and Silky Faille for slightly crisper images, but a little sacrifice in acoustical performance (only noticeable with measuring equipment and software). The SF is a stiffer material that is a tad bit easier to get nice and square, but if you need to stretch it at all you are kind of out of luck because stretching it actually makes some noticeable distortions. The PK is more stretchy which makes it a little harder to get square, but easily fixed to make it right. That only matters if you have something, like those move poster images, that need to be square. With artwork it looks cool to have the whole image wrap around the sides like a painter's canvas.
If you want just a plain color for cheap locally (Spoonflower is really cheap for what you get, but obviously more than a plain color), you can use a basic solid muslin fabric (a bit too "industrial" for me), or really any fabric that you can literally blow through... Try putting the fabric up to your mouth and think, "If I was a hostage, and they put this over my mouth, would I be able to breathe, or would I pass out."
If you can breathe through it, then you are pretty good for being passable for covering an absorption panel.
Just make a frame out of whatever wood material you want, take the fabric, lay it down, and warp the frame around to the back and staple it. Look for YouTube videos on canvas stretching for some good techniques.
Then put your insulation material inside.
Keep this in mind...
2" of material with a 2" air gap behind it performs almost as well as 4" of the same material but obviously the 2" is cheaper and the air is free.
I went with 2" of OC703 with a 1.5" air gap because I wanted to use readily available lumber and a 1x4 is .75" x 3.5". Works great.
Oh, and yes the Roxul Safe 'n Sound is a VERY common material to use for this application. You did good there!