So I am getting can lights installed in my Theater room and am planning on building backer boxes for them as i'm soundproofing this room. The space in my joist cavities is 9" high by 14" wide. Is this enough space to build said backer box for a 6" can light or would 4" be the only way to go. I'm doing at LEAST Double drywall + Green Glue for the ceiling and possibly furring channel/sound clips. Thanks.