acoustic panels

Hi, I want to make 4 DIY acoustic panels. I've read on reddit that the important thing is the thickness of the material, not have to worry about material type itself. But I have no experience, what should i choose? I like that polyester thing or rockwool 'cause of the thickness they provide...

Hi. I currently have a Genelec 8341A setup like in the photos below (the tweeters are perfectly at ear canal height): This is centered within the right half of a 27'4" wide living room, the left half being 14'11" deep and the right half where my setup is being 14'3" deep, the ceiling being...