Peek is a mod that allows you to preview the contents of shulker boxes.
It also allows you to add a label or an item on a shulker box.
There are a few ways it does this.
If there’s only one type of item in the shulker box,
that’s the item it will show on the lid.
You can, in an anvil, name a shulker box an item name like gold block or oak sapling and it will add the item to the lid.
You can add data strings in the name. These allow you to keep the box named to whatever you want, and have a label and/or item displayed.
The example in the documentation is My Box {i=stone;l=Rocks}; a shulker box named “My Box” with a stone block on the lid (and on the item in your inventory) and the text “Rocks” on the lid.