From: Rabbi Moshe Sofer
To: Rabbi Tzvi Hirsch
The Chasam Sofer rules that when the hollowing and fixing of a mikvah cistern occur simultaneously, the structure never attains the status of a detached vessel and the mikvah is unquestionably valid. He supports this ruling from the Rashba's teshuvah permitting rooftop mikvah construction and from an independent analysis of Vayikra 11:36 following the Ramban. He additionally cautions against a proposed partition arrangement requiring women to open and close a hashakah hole, citing personal experience with serious halachic failures in a similar setup.
Source: Shu"t Chasam Sofer YD §217