Look.
Oswald walked three whole aisles (crazy, I know) to get two of the smaller Rolling Readers boxes to use as the gun rest in the sniper's nest. Oswald was the only employee who's prints were on those boxes and the prints were recent. The floor laying crew would have no reason to move those boxes since they were not over on the west end of the floor (like they did with the bigger boxes containing books). The two Rolling Readers boxes, being three aisles away from the sniper's nest, had no business in the sniper's nest other than to be used as a gun rest.
Whoever shot from up there most likely had the boxes in place before Bonnie Ray Williams arrived. So the point that Williams was up there until 12:20 or 12:25 (which itself is debatable) and therefore created an extra risk for the shooter to move boxes around AFTER Williams left, is most likely invalid.
There is no photographic evidence of law enforcement personnel climbing all over boxes in the sniper's nest, despite what has been claimed here in this thread.