Unique= no fun

My control room is approx 14.xft X 12.xft X 8.xft. With sloped ceiling and doorless cavities for "side walls". I have just purchased new monitors and 144sq ft of Owens Corning 703. My main concern is what to do with the cavities in the side walls. Currently these cavities are filled with clothes, furniture and various other stored materials. But, I can sacrifice their storage purpose if they require acoustic stuffing/treatment. Another hindrance is that I am renting, so I can't go putting up sheetrock permanently. However, I do have a bunch of 4 x 8 sheets of drywall if they would be useful in a temporary setup. My plan was to build 4 corner bass traps and then treat the ceiling, front and back walls with the 703. But, I dont know what to do with the approx. 4x4 cavities because the insides are not cubed. The back walls of the cavites are curved and get deeper as the wall meets the floor. I was thinking of getting some Roxul and mimicing the Superchunks for these empty spaces, but I dont know if there would be a more cost effective solution. What if I framed some mineral wool or FG and placed it over the cavities as opposed to stuffing them. Super air gap?? Again I am renting so I want to be able to reuse whatever I buy for this place, but I am willing to spend. Here are the pics: