The hardened steel will still rust. Badly. You could use aluminum (maybe boil it first to build up a good oxide layer), brass (pickled to leach the lead) or nylon, if it is thick enough. You just need something to back up the o-ring so that it will compress and conform to the cooler wall and provide a good watertight seal.
Primary: , Boddington's Clone, Mirror Pond w/Marris Otter
Bottled: Fullers ESB Clone
Kegged: Mirror Pond Clone
Drinking: BM's Kona Fire Rock, Dead Guy Clone, Alaskan Amber Alt Clone, Firestone DBA Clone, Magnum/Crystal IPA
Next up: Burton IPA