I don't believe that non parallel walls eliminate standing waves. The shape of an enclosure will determine the natural frequencies and mode shapes of the standing waves but I do not know of any shape that is free of resonances. I think that the position of the driver, the port, and fiber fill for damping can be used to minimize the impact of standing waves on the back surface of the driver. A standing wave with a pressure maximum on the back of the driver's cone will tend to reduce the cone motion and cause a null in the driver's SPL response. Again, look at Figure 6 in the ML TQWT article.
Martin