That green stuff is also from my Uncle's backyard (lol). It is green shadecloth, it stretches a tiny bit but not much. It is some sort of synthetic weave. I used the staple gun and stretched it extremely tight across the face of the frame, so that it held the rockwool in place.
I purchased 5 white bed sheets from Saint Vinnies (like a second hand clothes store) for 2 Australian Dollars. I ironed them and stretched them across the frame, fixing them in place with the staple gun. And thats what got me to picture one at the top there.
Super Low tech. I've done that before. You can treat a large garage and a living room for a recording session for about $200 worth of material: Pine board frames with hard-board backers so they work as Gobo's too. Sturdy enough to move around for a while and look alright. The key is getting the fabric on sale... Get a little nicer weaves.