I am ~5’-9” and the seated position provides a comfortable position for the Z224 & Z313 shots.
I don't know how tall Secret Service agent John Howlett was, but the 11/27/63 reenactment photo shows him sitting on your beloved box (apparently in its correct position) and assuming a shooting position similar to what the "Oswald" actor does in "The Lost Bullet"
for the 2nd and 3rd shots, so I guess it's plausible that 5' 9" Oswald fired those shots while sitting on it with one bun on and one bun off, just like Howlett was doing as he was pretending to shoot.
More importantly, given the fact that we now know that Oswald fired his first, missing-everything, shot at hypothetical "Z-124," (half-a-second before Zapruder resumed filming at Z-133) and we know that the window was only about 1/3 open (going from memory here), that Oswald had to stand and awkwardly lean forward while firing the steeply-downward-angled shot, and that this is corroborated by the fact that in a digitally enhanced clip from the Robert Hughes film which is embedded in "The Lost Bullet," we can see something light-colored (LHO's t-shirt?) moving in the window about five seconds before the first shot rang (or banged, boomed, or popped?) out.