Pour the juice into a 20 cm x 30xm shallow metal tray and place into the freezer for 4 - 5 hours, scraping with a fork every hour or so to form crystals (thus preventing it from freezing into an ice block). (theholisticingredient.com)
Harry retreated to the Headmaster's office to use the Pensieve, a stone basin with a detached shallow metal dish, to examine Snape's memories. (filmsite.org)
(Or rumor has it you can pour the mixture in a shallow metal pan, and freeze overnight. (takeamegabite.com)