Note: Feel free to edit the title of this question if you know of a better way to put it.
I got a new bathroom vanity (cabinet and sink top). In spite of its high price, the quality is not so good. It's rectangular in width and depth, but it skews like in the picture below when any kind of pressure is applied to any of the sides.
I would like to lock it into shape somehow so that it doesn't move. Even with the sink top on, it still moves. I can tell because the edge of the sink top squeaks against the wall behind it. Rather than fix the symptom by putting something like a rubber strip along the back, I would rather tackle the actual problem. I imagine there is some kind of sturdy metal brace that I can screw in to each of the corners.