This video is why I made the assumption SW's body would not fit into the opening. I'm pretty sure this video does not depict the actual storage tanks that are on the site where CW hid the bodies of his two little girls.
JMO, I definitely think CW planned everything out and given his job, he should have accounted for the size of the opening and whether SW's body would fit. Or one would think.
Perhaps the openings on the tanks on that site are larger than the ones in the video and SW's would fit but for some reason, he was unable to get her body there?
If this turns out to be the case, I can think of two scenarios off the top of my head of why he might not have been able to get SW's body to the tank opening.
One might have been physical exhaustion. Another might be that someone was now within a distance where CW feared they could see him carrying SW's body up the stairs to the tank opening.
IDK! Hopefully one day we will know. It is JMO that SW's grave was something CW considered to be temporary and he planned to come back later because I can't think of one reason why he would want for SW's body to be discovered.
All JMO.