From: Rabbi Moshe Sofer
To: Rabbi Meir
The question is whether a tzuras ha-pesach for a mavoi is valid when its vertical reeds are positioned behind the wall and invisible from within. The Chasam Sofer distinguishes: when the crossbar is above twenty amos and the posts are also invisible, the structure lacks all recognizability and is invalid; but when the crossbar is visible below twenty amos, the hidden posts do not disqualify it.
Source: Shu"t Chasam Sofer OC §91