The plug mentioned that does not have shoulders is sometimes known as a Welch Plug. They can be found with some searching. Below is a good description for installation.
Good luck with your Polar Vortex victim. The night before our temperatures plummeted I hurriedly checked the coolant condition on all of the machines. Who would have thought temperatures would have dropped so low.
http://www.hubbardspring.com/install_reco.php?cid=45
