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
Those darn SS washers are hard to find. Skifast1 actually bought a bulk order and is selling them cheap (his cost, I think). Search Members for him, and he has a link in is sig about buying them from him. I think he is selling them for a buck each, shipped.
I finally got my 5/8 ss at place here in Alaska call Alaska Industrial Hardware and they were closing so they thought they were just steel I guess and they said take them they are yours along with some O ring to fit too.
Sometime things just go your way.
The price is right and no leaks!!