Trim the green skin off 1/4 small watermelon. Remove all but 1/4 inch red fruit from the rind; cut the rind into 1/2-inch pieces. Pack into a 1-quart jar. Combine 3/4 cup each apple cider vinegar and water, 1/2 cup sugar, 2 tablespoons kosher salt, 2 teaspoons coriander seeds, 1/2 teaspoon black peppercorns and 1/4 teaspoon red pepper flakes in a saucepan and bring to a boil, stirring to dissolve the sugar. Pour over the rind; let cool. Cover and refrigerate at least 4 hours and up to 1 week.