The MDX crossbars are 3.5" wide, and they are spaced approximately 31.5" apart, and, of course, they're not adjustable.
I don't know about the xCargo mounting, but my Thule Classic roof box is designed to mount with clamps, rather than the bolts that advarkus identifies. As obeejr describes, on the Thule you'd need to put reinforcing plates on the inside to diffuse the stresses from the bolts (the clamps already have such plates, and use two bolts per fix point, rather than one).
Also, if you don't use clamps, I think the bolt holes would need to be placed precisely at the spacing of the MDX cross bars. The clamps are wide enought to allow some flexibility with regard to spacing. For the Classic, I had to drill two new mounting holes outboard of the existing holes, because the box did not allow the clamps to be spaced at the 31.5" that the MDX requires.
Also, note that the cross bars are bowed, rather than flat, which may affect how your box is mounted.