I have used wire for about 40 years. A good quality wire is safe. Obviously if you have a strong dog who is determined to break out, you may need to go to the uber expensive crates that are almost solid metal. Few dogs need those. As Pat said, no top means no dog. They laugh at 6 feet, much less 40 inches. They can jump or climb easily. Plastic ones are more easily chewed unless the solid ones... which block view and air.
