I did sort of the same thing for my Wittes ride from Seattle, Washington to Kailua-Kona, Hawaii. I used 4x4 inch landscape treated "timbers" for the frame (you can't get ground contact rated wood here in Hawaii (Hey, I don't make the rules I just figure out to beat them.) And built the box out of, you guessed it, 1/2 inch treated plywood. I used the plywood for a new floor in the coffee shacks bathroom (third and final layer). The box was also oversized because I wanted to use as much uncut pieces as possible.
The two main differences from my box and yours are that I used the 4x4 bottom frame as the built in pallet. My shipper included the pallet in determining the cubic dimensions which is the basis of shipping charges to Hawaii and the other small point is that I crammed the box full of really import crap that would have cost more to ship than the 2600 pound box marked antique generator - Mr. Happy weighs in at less than 1500 pounds. Oh, around $700 port to port.
Casey