Rename your shulker to any model and render an icon on your box
Rename your shulker box in an anvil to the item that you want it to show
NOTE: This only supports all models NOT Model Definitions. For a full list look here
Example:

To Build just run the build.py
Note: I have only tested this on linux but it should work on windows. An internet connection to the mojang servers is requierd(for the assets)
python ./build.py
This will generate all the models and item definitions