Miuccia Prada is a tough audience. The designer was here yesterday designing costumes for the Metropolitan Opera’s upcoming production of “Attila,” Paper magazine reports, and she met with some of the women who were cast months ago as extras. But Prada didn’t like their curvy figures. “I cannot clothe them! I need models!” she reportedly groaned. Paper claims the Met is now on the hunt for models. A rep for the opera company told us that casting “is at the discretion of the creative team. Due to a change in concept, the Met is in the process of recasting” some of the extras.