I'm building a set myself at the moment. Imagine a wedge shaped box that allows the back seats to recline slightly. It goes most of the way across the back, except for the spot for the water cooler.

Of course as far as I've got is the basic shape. It's made of 12mm board. The plan is to build in shelves to suit the stuff we want to put in it. In my case it is our kitchen stuff.

For inspiration, check out some drawer, kitchen, camper van builds. Websites like drifta. Find an idea here or there, and apply or adapt it to your situation.