Unless something better has come along, people used to use Shoo Goo on the inside of the body to help cracks. There's nothing you really can do to completely heal it. The goo will basically flex while running, but still hold together to keep the crack from getting worse. If you want to add a little insurance, drill very small holes through the plastic right at the very ends of the crack. The trued edges of the drill holes will keep the crack from spreading any further.