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).
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.
(Or rumor has it you can pour the mixture in
a shallow metal pan, and freeze overnight.
(Alternatively, freeze the mixture in
a shallow metal cake pan until solid, about 6 hours.
If you don't have an ice cream maker, freeze the mixture in
a shallow metal cake pan or ice cube trays until solid, about 6 hours.
Place
a shallow metal or cast iron pan (not glass, Pyrex, or ceramic) on the lowest oven rack, and have 1 cup of hot water ready to go.
The second time I used
a shallow metal baking pan and baked them on convection for longer time.
Keep clutter in control without overwhelming a narrow space by adding
a shallow metal rack.