I thought you tuned your box about 5 hz or so lower than the resonant frequency of your vehicle? You could use winisd or Products Menu
to figure out how long the aero needs to be. To account for displacement it's .5 of the OD of the aero squared times pi times the length of the aero (don't forget the flares).
V =(Diameter/2)^2 x pi x length /1728