Your comments

Unfortunately, you can't. These Boxshot models are static files, not intended for modifying. I would recommend to find a third-party shape you need on sites like and modify it the way you need.

We'll probably provide more possibilities in the "boxshop" area of the site, which we are going to add soon, but at the moment a third-party shape is the only solution.
What if you assign the same glass material to the label?