What do I need to install a Thule box onto the factory rack? Or do I need to purchase a mount.

I have bought it used from a friend.
Any advice?

Here is photo of box:

I beleive the vertical Mighty Mounts that wrap around the stock rack will work. I would reinforce the inside bottom of the carrier where the bolts for the Mighty Mounts would go through. Four peices of flat aluminum about 3 inches square that are drilled for the bolts should work just fine. You dont want that carrier to fly off while your driving.

My Thule box (Cascade) mounts directly onto the factory rack. The only modification required was to swap the Thule nuts with a lower profile (skinnier) wingnut. The bolts are a little too short to get around the X's crossbars and into the Thule nuts. Other than that you should be fine, assuming the box came with all of its mounting hardware.