Example usage of ItemsAdder API to crete custom textured inventories in Spigot
I know the code is not perfect but I just wanted to show a basic API usage, you're free to copy it, edit it and include in your plugin.
TexturedInventoryWrapper inventory = new TexturedInventoryWrapper(null,
54,
ChatColor.BLACK + "Test",
new FontImageWrapper("mcguis:blank_menu")
);