Keep frozen. Using a melon baller, scoop out 1 1/2 cups watermelon balls (and cantaloupe balls, if using) into a small bowl. Cover and chill. Cut enough watermelon into 1" pieces to measure 8 cups.

Scoop out the flesh with a melon baller, creating as many balls as you can. Remove the rest of the flesh but leave a slight border around the inside. Drain away any juices and discard the seeds.