I mounted 2 ATV boxes very similar to the ones above. Here is a link: http://www.truckcustomizers.com/prod...-tool-box.html
I purchased them locally from a truck accessories place.
Here are a couple of pictures. I started off with one and then added the second. I used 1/4" thick 2" by 2" aluminum angle bolted to the bottom flange of the trailer I beams.
They hold alot and I keep my jack, tools, boat cleaning supplies, spare trailer parts in one and then a pressure washer and hose in the other when I am gone for awhile to wash the boat and trailer.