Plasma is the way to go. You should be able to rest the torch head right against the inside rim of the keg and use it as a guide, this will leave a nice 1/2" or so lip around the top that will act as a slosh baffle, if you have the tank on wheels. In a previous life, I owned the first "Air Plasma" in South Florida that would cut 1/2" material. South Florida humidity + compressor = very wet air, as you said, not a good match for a plaz. I had a drier that was attached to the plasma only, a canister with an inlet and an outlet, and the filter media was only a roll of toilet paper. Cheap and readily available to replace. It worked great for me, I am sure with a little looking on line, you could find one. I probably got mine from a welding supply, but can't remember a manufacturer name. It is only about 13 gauge or less thick, so even with a ratty cut, it is an easy grind to clean up.
Cheers, Mike L. .