You can substitute a 10.5-ounce loaf angel food cake for a round cake. Slice off top third of loaf, using a serrated knife. Hollow out bottom portion, leaving a 1/2-inch shell (make sure not to cut through bottom of cake.) Proceed with recipe as directed.
Slice off top third of angel food cake, using a serrated knife; set aside. Hollow out bottom portion of cake, leaving a 1- inch-thick shell, making sure not to cut through bottom of cake; reserve leftover cake (about 1 3/4 cups) for another use.
Combine ice cream, lemonade concentrate, and 1 teaspoon lime rind. Spoon ice cream mixture into cake shell, pressing firmly with back of spoon; top with remaining third of cake.
Combine whipped topping and remaining 3 teaspoons lime rind; spread over top and sides of filled cake. Cover loosely, and freeze 6 hours or until firm. Slice with a serrated knife.