Actually Jack, based on the layout of the fairgrounds, stated distance of the shooter by the SS and photos of the shooter's body on the roof, my guess is that the shooter was omn the roof of one of these two restrooms.
View attachment 977495
As restrooms, they would have remained open and available to anyone, and a guy walking into it would not have drawn any attention. There must have been a door or something leading to the roof. I'm not sure the SS could have done much to stop this except maybe finding these doors and locking them, but as you can see, there were a lot of buildings and maybe they needed access to the roof left open for another reason, or maybe the kid went up there hours earlier and broke the lock.
My only complaint is that in the future, they should not use a place (and normally don't) with so many building around from which a shooter might shoot from, but then, most assassination attempts happen in close range using a handgun. But you can bet it won't happen again.