to put boxes under the cat, you not only need clearance under the trampoiline, but clearance for the dolphin striker during loading & unloading. Otherwise, you have to lift the front part of boat to clear dolphion striker over your box. There is usually not much vertical distance between bottom of hulls (where they sit on trailer rollers or pads) and bottom of dolphin striker.
I mounted a cross-bed style truck box parallel to sail tube, to get extra storage. This type :
http://www.contico.com/automotive-blow-tb.htm