The 2x2in corner posts provide firm footing for the cabinet, and secure mounting for the shelves.
For both stability and appearance, I used 1x6in boards, biscuit-jointed together
and planed, for the backplane of the cabinet. Many commercial manufacturers use
hardboard, or even cardboard(!) on a piece this large to cut costs and raise profits.
Here is the cabinet, ready to be stained.
The finished product, installed and in use.