Actually the Fb will go down when you put the box into a car. The effect is similar to a dual chambered alignment, with the volume of the car interior being the second chamber, but the effect is dwarfed by cabin gain to the extent that it is rendered moot. Standing waves and room gain aren't the same, and you don't get both simultaneously, as one requires less than 1/2 wavelength room dimensions while the other requires at least 1/2 wavelength dimensions.