This is a very difficult shape to locate when it comes to terracotta pottery. It is hard to manufacture a square corner and there tends to be a lot damage in the firing process. That is what makes this shallow square so popular. We found these in Mexico and they are made of real clay that is fired in a kiln. The sides are smooth and compact and the corners sharp.
Don't hesitate to pick one up if you are looking because these are not always available.