I want them to be so thick that they would stand on the floor by themselves so yeah, 6-8" easy, more like 30 cm which is around 11" I think
I'd be looking at Safe n Sound or even attic insulation (pink fluffy, as it's generically called) if you're talking about that sort of depth.
Go here to play around with absorber depths and gas flow resistivity (GFR) values to determine the best GFR for the depth you want. The thicker the trap, the lower you want the GFR to be. GFR kind of sort of correlates with density, but not exactly, so the lower the density, the lower the GFR tends to be, but you still really want to choose products based on demonstrable GFR values.
Then go here to see what product has the appropriate GFR, that is available for purchase where you live. On the last few pages of this thread, is also a few posts where the relevant spreadsheet is updated and linked, where the everything is listed.
Curious -- are you going to cover all your walls with the panels?
I want one on the front wall (yuuuge window), in the front corners and on the left/right. There is a huge wardrobe on the back wall, it will be filled with soft stuff and the doors will be left open, so it will act like a big-ass bass trap
You have a good plan, linas, and YES cover them! Two layers of a durable cotton blend upholstery fabric from any local fabric/sewing outlet store. There's always one with 5 miles of a retirement home as old ladies love to sew! (and so do I even though I am neither old nor a lady HAHA)