Inspired by an expensive model on Etsy, I decided to draw a cup holder for the BMW Z4 myself. Holds two cups or bottles and a bit of small stuff. Print in PETG or something else that won't melt in a car on a hot summer day.
I would suggest to add a bit of velcro to the back and the bottom of the door to the boot (see pictures). Not needed, but makes the holder extra secured and won't rattle.