I've just completed the installation of a undercarriage cabinet, 40"w x 9"h x 11' long. It terminates in front of the axle, and carries a port note (a folding boat).
Not wanting to stress the already flimsy frame, I fabricated some .125" angle iron with mounting plates that bolt into the I-beams. From that I hang the two 11' trusses, and bolt angle iron across the bottom every 30", used .375" ply on the bottom with a laminate flooring to reduce scratching, and used coroplast for the sides.
Hope this helps.