Someone stole your hitch or the receiver? If they snatched the hitch and ball just pick a new one up. The factory hitch kit on the X5 has a 2" receiver. Any generic 2" ball and slide will fit the factory hitch or pick up a new genuine BMW one
HERE.
I've as for a king mattress strapped to the roof of an X5, I've heard of stranger things, just make sure you use several tie down points across the front of the matters to prevent it from lifting at highway speed.