I got tired of knocking over my drink with the stock cup holder and dislocating my shoulder to reach the cup between the head rests. Here's my simple solution...

Start with a standard cup holder, designed to fit on your door window. I shaved the back of the attachment part so that it was a thin as possible while still being structurally sound.

Then I heated the hook and shaped it over my glove box door. The result is a functional, out of the way cup holder that can be removed when not needed.

Let me know what you think

Hook was thinned using a dremel cutter to eat away the plastic a little at a time and yes it holds a large milk shake without problem.