If a Yakima box/rack/etc uses mighty mounts (horizontal or vertical) then you should be able to use the Xterra rack (like Evergreen says use xterraparts.com). I believe the Xterra rack is rated to 150lbs (maybe 125, but that's for laywers) which is also the weight capacity of the cargo box you're looking at.
The only problem I've found with the mightymounts is making sure you get a tight fit on the Xterra crossbars so that your box doesn't shift under load. But I'm using a Kargomaster steel rack and have sort of retrofitted the mightymounts to work with it.