It’s a question of when Oswald would have read the newspaper
If it is not until Thursday, then the reason is it’s risky to take a rifle in a package into Dealey plaza on Friday morning,let alone into some other building
On the other hand, if Oswald read the Tuesday paper then he had more than enough time to explore Dealey plaza, select some spot or building, and to carry a package with rifle. If William Castor could walk around carrying TWO rifles as well as EXHIBIT them without any DPD , FBI, or other security being alarmed, then surely Oswald could have probably done so with just ONE package
It would have been impossible to check every person who entered a building along the parade route. Imagine the resources to do that in every city the president visited? That would have entailed securing hundreds of large buildings and tens of thousands of people. An impossible task. They certainly wouldn't have done that days before the presidential motorcade when Caster brought his rifles. Assassinating the president entails risk. But getting the rifle into a building in which he worked wasn't that difficult. Oswald knew the movements and patterns of his coworkers, likely had a hiding spot and shooting location predetermined. That morning he goes straight up to the 6th floor and hides his rifle. My guess is that he hid it near where his clipboard was later found on the 6th floor. The last thing he did before picking up the rifle package was to put his clipboard down. He used his clipboard to give the appearance of doing work on the 6th floor in case he encountered anyone while he was lingering up there. Once the coast is clear he puts it down, picks up the rifle package, and makes his way to the SN. Frazier was likely his biggest concern. He would have known Oswald's movements were out of the ordinary that week, that he carried a long package to work, and that the presidential motorcade was passing the building that day. He had more pieces than anyone else but at that time it was unthinkable by most people that anyone would shoot the president.