ChestLocker

As simple as it gets:
Drag it into your plugin folder and that's it. No configuration needed.

What makes it different to other chest locker plugins:
It's extremely lightweight:

"ChestLocker by earomc" uses the PersistentDataContainer API. This means there's no database or extra plugin-created files that store block data.

Functionality:

This plugin allows you to lock pretty much every item container in Minecraft including chests, barrels, droppers, dispensers, shulker boxes, furnaces, blast furnaces, smokers, brewing stands, beacons and hoppers. Single and double chests are supported.

Locking a container:

To lock a container, you just need to type /lock <password> while specifiying a password. Then, right-clicking the container will lock it.
After the container is locked, you can only access it with an item that has the exact same name (case-sensitive) as the password you used. That means you need an anvil to rename an item according to the password you chose.

When a container is locked you cannot break it, blow it up, use hoppers or hopper minecarts to get out the items. Additionally, shulker boxes cannot be destroyed by pushing them with a piston when they are locked.

Unlocking a container:

To unlock a container, use /unlock <password>. You need to specify the password that was used to originally lock the container.

In case somebody forgot the password of the container you can unlock it via /adminunlock

If you want to abort the action of locking or unlocking a container, you can just sneak and it will abort it.

Permissions:

The permission node for the /adminunlock command is
chestlocker.adminunlock

Bugs / Issues:

If you run into any bugs or issues while using the plugin, or have any suggestions to improve it, please feel free to use the Issues tab on this plugin's github page.

If you like this plugin, please consider giving it a star. Thank you very much! :)