Special equipment: 1-gallon wide-mouthed beverage container Remove the rind from the cantaloupe and honeydew, and cut the flesh into 1-inch chunks (you should have about 4 cups of each melon). Put the honeydew, cantaloupe, blackberries and red grapes on a rimmed baking sheet in a single layer, and freeze until frozen completely, like ice cubes, 4 hours up to overnight.
Meanwhile, whisk together the lemon juice, cranberry juice, grape juice and 1 cup cold water in a 1-gallon container with a large enough mouth to accommodate the fruit chunks; refrigerate while you wait for the fruit to freeze.
When ready to serve, add the frozen fruit to the juice mixture, and serve immediately or pack it up for a summer outing.
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