That is going to look good there and with another beside it too.
You have a piece of frame there to work with for bracket building.
You also have the open webs across the bus width.
Ok this per box
3 pcs of 2inch flat bar with a curl on one end that will go over the beam and extend to the approx front of the box, or wherever you want for length.
In the curled end, short side, drill and tap for a 1/2" bolt that will tighten up against the inner frame from underneath. They won't go anywhere then with the weight in the box.
This just seemed easier than welding or drilling brackets off the frame. Slide these in, tighten, done. Except for the actual bolting of the box to these 3 pcs. Two pieces spaced evenly might work but 3 will definitely make you feel better about the mount.