There are a bazillion tool boxes and types. Google them, find the one you want that will fit, then google where to get it cheapest. Look around.
I just took one off of my wife's truck (Yes, my wife drives a truck!
) They come with little u-bolt type things that are made to go through the bed stake holes of a pickup. But they might work for the rails of the trailer frame.
If not, and if this thing is going to stay on forever (I assume it will) then I wouldn't hesitate to cut two holes in bottom of tool box to match trailer rails, cut holes in them too, bolt on with Grade 5 bolts and HD fender washers, and cover the bolts and the top and bottom with good marine silicone, and call it good.