Watermelon Ice Cream Slices

Whisk together the condensed milk, salt and reserved watermelon juice in a large bowl and set aside. Whip the heavy cream in a large bowl with an electric mixer on medium-high speed until firm peaks form, about 2 minutes.

Fold about one-half of the whipped cream into the condensed milk mixture with a rubber spatula until combined, then fold the lightened mixture into the remaining whipped cream until well blended.

Pour the mixture into the frozen watermelon, filling it all the way to the top. Cover with plastic wrap and freeze until solid, 3 hours and up to overnight. 

https://www.foodnetwork.com/recipes/food-network-kitchen/no-churn-watermelon-ice-cream-slices-5220055