They just need a Spout Pouch
We have already solved this problem for situations where we need to reliably contain liquids while still being able to access them, particularly spaceflight. You need to make the metal thin enough that it becomes flexible, while attaching it to a substrate sturdy enough to prevent it tearing.
While we usually use a plastic substrate coated with aluminium using vapour deposition, even a medieval society could have achieved something similar with animal bladders and chemical deposition. We would certainly have had ready access to this tech in 1998, although if your magical-stasis metal was not aluminium they would require some custom production runs.