truck bed box

I mounted a truck box on the tongue of a flat bed utility trailer I had. The box was the heavy-duty ABS kind, not the diamond plate so it needed full support underneath. I used two 5' pieces of 12-gauge 1-5/8" Unistrut channel from Lowes/Home Depot. The channel was bolted to the tongue with square u-bolts. The nuts were in the up position and concealed inside the channel. The box then bolted to the channel through the box bottom and I used the unistrut capture nuts. All the hardware was concealed nicely. The tool box stayed with the trailer when I sold it a year later (for a tidy profit!). The new owners of the trailer love having the box; they use the trailer for a snowmobile and the box houses their support gear for it.