Quick update: bought a pair of bar magnets for $2.79, tied off to one and placed the other on the outside of the lid. In business.
I may try the floss next time, as I like the simplicity of having something just protruding through the lid.
I suspect that it sealed during my dry run because the weight of the ball wasn't there to pull on the seal. Even if a perfect seal isn't necessary till carbonating, I'll sleep better now!
Thanks for the replies, folks.