Low Flier, what makes you think they are looking for the box?
About 30 years experience of seabed search and recovery work looking for such wreckage while working as a civilian contractor for organisations such as USN SupSalv and as a client representative on similar jobs outside the military sphere. It is the two data recorders which will provide the most important evidence for the investigators.
120kHz gives a resolution of about 1500/120000/2 (half wavelength) or about 6 millimeters.
In your dreams, JD! I promise you that you will never ever find or resolve a centimetric sized object with a 120kHz sidescan sonar. I've never knowingly looked at a CVR-sized box with such a low frequency sonar, but I suspect that it'd be quite difficult, and more probably impossible, to find at such a low frequency.