What is the minimal amount of clearance from the top of the box to the trunk top for an upfiring box? I've got 14.5-15" MAX trunk height to work with for my two 15" Avalanches coming in. My plan was to put them in a box 11" high (9.5" internal height), leaving me ~4" in clearance for the sub to move. I realize the sub won't be moving that far, but I want to make sure I won't be hindering performance by having that little space above the driver. Anyone know about this?