Here's a couple of pics I took this afternoon, of the area where the box will go on Burb. The box is interchangeable with the auxiliary battery tray, and attachment is the same.
I found that I do have a set of the nut and bolt for the box attachment, that's original GM hardware (used of course) and I went to O'Reilly's and they have a service replacement. You have to buy the nut and bolt, and the metal speed nut that goes in the fender, separately though, but you do get a few of each piece. I thought I had hardware for each storage box I had to sell, but only found one set.