I went down this road recently and have done a fair amount of research on the subject but by no means am I an expert. A few things I walked away with. 1. It depends on the size of your room. My room is pretty small, so it is recommended to trap as much Bass as you can. 2. 4" minimum is required to capture much base at all and it doesn't discriminate, so you need to use some sort of deflector so that it doesn't absorb all the mid and higher frequency (which is what makes your room sound dead 3. there is very little difference between Rockwool, Fiberglass insulation and the different densities of those products.
I made some 8' corner base traps using 2' x 4' x 7 1/2" that I found at Lowe's. Generally the big box stores don't carry much in the 2' wide size. It's usually the 15.25" x 3 1/2" size which fits between standard wall studs which is a little shy on what you need for depth. I opted to be that from ATS as mentioned above.