I have a couple “wet boxes”. These are plastic bins with lids into which a layer of plaster has been poured. The plaster is kept wet in order to maintain humidty, slowing (if not stopping) the drying process.
However, I haven’t used the wet boxes in a long time. I’ve found it much easier and more convenient to simply wrap each piece in it’s own plastic trash bag. Pieces stored in this manner can be trimmed weeks or even months later.