From: Rabbi Moshe Sofer
To: Rabbi Wolf
The Chasam Sofer provides a comprehensive plan for constructing a valid community mikvah, ruling that rainwater collected via metal gutters and smooth wooden channels near the mikvah is the ideal water source. He further permits filling with snow and ice carried on one's shoulders when rain is delayed, and strongly recommends building two adjacent pits with a breach near the top of the dividing wall — a system already in use at the main Pressburg mikvah — which ensures a perpetual supply of kosher water and allows cleaning without ever invalidating the mikvah.
Source: Shu"t Chasam Sofer YD §203