Max Holland’s video depiction of the sniper firing the first shot from a standing position is lacking a lot. If he had constructed a complete sniper’s nest with all of the boxes in their proper places (including all of the boxes immediately surrounding the nest), he would have found out that there really isn’t enough room for a sniper to kneel between the boxes.
Sitting on the box while firing all three shots would have been his most stable and feasible position given the ergonomics of the sniper’s nest, and he should have known that from his military training and experience. The window box was in a position that could have interfered with an early first shot. That interference might have caused an inadvertent discharge (before it was fully aimed).
Holland did recreate the Sniper's Nest accurately. Watch "The Lost Bullet."
The "sitting box" wasn't close enough to the window to permit him to sit on it while shooting. To sit on and lean forward every minute or so to see if the motorcade had turned onto Houston Street, yes, but not to shoot from.
Do you think Oswald used the stack of boxes at the window at all while he shooting?
If not, why did he go to the trouble of putting them there if he wasn't planning on using them?
The first shot at hypothetical "Z-124" was about a second later than Holland thought it was (at hypothetical "Z-107"), requiring Oswald to swivel a little bit farther to his left than Holland shows him in "The Lost Bullet."