A vanilla-friendly backpack variant: Bundled Shulkerbox

Idea: A shulkerbox can be put in a bundle.

1. Obtaining.
- Sneak + Rclick with an empty bundle on a shulkerbox to pick it up
- Merge shulkerbox with an empty bundle inside a crafting grid
- Sneak + Rclick with a bundle while holding a shulkerbox in an offhand

2. Usage
Ctrl + Lclick (or maybe rclick) will open its GUI.
The GUI should be additional and from my perspective it would be fine to open somewhat a 90 degrees rotated shulker menu to the left or right from the main inventory: (a prototype)
The shulkerbox can be placed by Shift + Rclick, leaving a bundle in your hand.

3. Texturing
By default it should replicate a shulkerbox model -- be a 3d model with some bundle-like overlay (which covers ~1/4 of the block). This, however could be changed to use Item texture.
No matter how many D's there are, there should be an option to texture the bundle overlay separately or texture the whole bundled shulkerbox. This needs to be stated in manifest.json.

Some info could be added later.

I wanted to post this to the backpack discussion, but it was closed.