From: Rabbi Moshe Sofer
To: Rabbi Moshe Kanische
The Chasam Sofer approves the proposed mikvah construction — piercing a barrel to nullify its vessel status and connecting it to a natural spring — as a praiseworthy stringency. He distinguishes between two scenarios: a hole the size of the vessel's purification measure suffices to prevent disqualification of the mikvah, while actual immersion within the vessel requires a hole the size of kishfoferes hanod, following the Ra'avad's reasoning.
Source: Shu"t Chasam Sofer YD §215