You're probably better off filling with fluffy especially with the depth you have. How high is it to the ceiling? The greater the distance, the more effective but anything will do something. Measuring with REW before and after will help you see the improvement. Using kraft paper faced fluffy (only on the front) will enhance LF absorption and reflect some mid/hi freqs. This you probably know is generally recommended for bass traps. Of course you'll probably want to install RFZ panels too.
thank you for the response Rock ! ok,, sounds totally do-able.
I have 3 long shelves which I could fill. They are all at the ceiling, which is 8'. The spaces are 18" x 12" x 6' - 18" x 12" x 8' - 12" x 12" x 16'
I just rearranged most of my rigid panels yesterday to make better use of them as "corner traps" at the available wall-walls, ceiling-wall, and floor-wall. They are all 4"x24"x8' rigid wool traps I built several years ago. The sonics in my room improved tremendously just from this. I haven't tested , so I don't know what my trouble frequencies are right now, but it is getting much better. I purchased a measurement mic, so I will be testing more accurately soon.
My room dimensions are in my signature, as I figure that's awfully important for these conversations, and I only have one room . I have (2) 2"x24"x8' RFZ panels above the drums, and (2) 4"x24"x8' RFZ panels further back on the ceiling. All hardwood floors.
Room: 24x19.5x8 (without bay window) Total cuft= 3996 (including bay window) (bay window at far end = 252 cuft) floor to ceiling built in shelves on a short wall and a long wall