Freeze copper line to stop flow

I would like to freeze a 1/2" "M" copper water line with dry ice to add a 1/2" valve 6' down stream. Anyone ever do this? How much of the line do you apply the ice to? 2"? 4"? 6"? How long? cloth around line first? Obviously over freezing would cause the line to split. Not good! I know there can be no water movement while this operation is being attempted..... When I was an apprentice in 1980 I was with a journeyman that froze a 3" "L" line in a hospital and it worked! I know they have commercial kits designed for this, but I would like to try this if I get some good answers here. The reason I can't just shut the main valve is I'm on the bottom of 4 story 32 unit resort condo and it's a lot of hassle to schedule a shutdown.

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