I have this already heavily customised wooden shelf for the dog's bowls. Unfortunately it didn't stand straight and there was some space between the wall and the upper part - enough for the dog to fling pieces of food behind it somehow.

Pieces of wood and cardboard propping up the shelf worked for a while, but they shifted over time and had to be stuck in again quite often. I needed "shoes" for the shelf that were just the right height and wouldn't allow for the shelf to move anymore.