It should fit snug to the stock rack, BUT!! Mine was just on there too, not loose, but not tight. If ya pushed on it, you could slide it and wiggle it.
I did not like that.
Underneath you can see exactly where the bottom of the box sits on the rack. I added strips of 1/8" firm rubber, like inner tube material, not foam.
This made it fit VERY firmly in place.
I like it on my RS for trips, but take it off around town.