"Lousy" plumbers do it all the time, because it is a ''quick and dirty" way to do it. Sometimes they use tar to seal a piece of tin over the hole, and other times they just lay the tin on the pipe to try to keep the dirt out. Your second plumbers were not much better than the first ones. I dig up the line, clear the line and then install the cleanout. I don't know how they expected anyone to install a cleanout if the pipe was full of water. Find a good plumber, one without the words "Rooter" in the name and not one that advertises on TV.