Copper doors and chests can be locked using renamed trialkeys.

The copper doors and chests were incredible additions, but I feel that, like the iron door, they could have had specific uses. *If you rename a trialkey and a copper door (or chest) with the same name, the two will be linked and you can use that key to lock the door and the chest, allowing it to only be opened using the trialkey. *Locked chests, when broken, will not drop items but will remain inaccessible until placed on the ground and unlocked. (Or, if you don't want to do that and don't want to surpass the Shulcker Box, you could make the chests locked unbreakable) *Locked doors wouldn't open even with redstone connected and would only open if a key was inserted by a player or a dispenser. *Not even the copper golem can open locked chests, but if it picks up a key corresponding to a nearby chest, it will become curious and may open it. This adds a whole range of possibilities for copper doors, whether for storing treasures or even playing pranks on friends.