Kitchen storage help

We have just moved into a new (RENTAL) house, it's an open plan kitchen living area and what you see is the only kitchen storage. I still have stuff in boxes and the cupboards are full. I am thinking of building a shelf (red drawing) that can rest on the top of the existing cupboard so it doesn't need to be screwed into the wall.

Will this look horrible? Any other ideas to increase storage? There seems like a lot of waisted space about around the fridge but I'm not sure how to use it.