I’m considering putting a homebrewed piece of treasure into my game (5e Exandria campaign). Specifically, it is a box, approximately the size of a breadbox. Time does not pass inside the box while the lid is shut. If the box is broken, the enchantment is lost. If a portal to an extradimensional space is placed into the box, such as a bag of holding, the interior of the extradimensional space is not affected, but anything inside such a space would be unable to leave while the box is shut.
My question is this: What potential issues or cheese might result from this item that I haven’t yet thought of? The party is currently lvl5.