There are holes cut in the back of the box and it's chained to the factory metal tie down loops. Similar to the setup in the front holding the ice scraper in place, but with a chain instead of the red bungie cord. I'm not sure how strong those loops are or would be in a crash, but I do hope I never have to find out. The bungies are just on there to help stop the front of the box from bouncing and making noise (since the front isn't held down any other way, just the back).

There is really no way to directly bolt it down since the spare tire sits under the box and I would have to unbolt the box to get to the tire.