I have recently seen a few posts that seem to indicate that LHO had to have assembled the SN in the last few minutes before the motorcade arrived. However in the Bronson film segment, which was filmed during the epileptic event at approximately 12:15, we can see that the window ledge already has the boxes on it. Here's a copy of the SFM image in Robin Unger's Gallery:
And here's a link to the larger image in Robin's Gallery:
https://www.jfkassassinationgallery.com/displayimage.php?pid=12705&fullsize=1
This is evidence that the SN was built before ~12:15. We have no way of knowing how long before ~12:15, but it does appear that it didn't need to be built at the last minute...
Givens places LHO on the 6th floor after the floor laying crew left and before BRW returns to the 6th floor. Plenty of time to stack a few boxes.
Mr. GIVENS. Well, it was about a quarter till 12, we were on our way downstairs, and we passed him, and he was standing at the gate on the fifth floor.
I came downstairs, and I discovered I left my cigarettes in my jacket pocket upstairs, and I took the elevator back upstairs to get my jacket with my cigarettes in it. When I got back upstairs, he was on the sixth floor in that vicinity, coming from that way.
Mr. BELIN. Coming from what way?
Mr. GIVENS. Toward the window up front where the shots were fired from.