Freeze the seltzer flat: Freeze in a thin layer if you're in a rush, to speed up the process.
Use fully frozen fruit: This keeps the texture thick and icy. Frozen strawberries are readily available in grocery stores, but frozen watermelon chunks can be more difficult to come by. To freeze your own, line up the fruit on a foil-lined baking sheet so that the pieces aren't touching, then freeze overnight.
Taste before serving: Adjust sweetness or add more rum if desired.
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