Freeze the evaporated skim milk in a 9-inch square baking pan, covered, for about 1 1/2 hours, or until slushy.
Combine the egg yolks, lemon rind, 1/4 cup lemon juice, and sugar substitute. Beat with an electric mixture at medium speed until smooth and set aside.
In a large chilled bowl, combine the partially frozen milk and the egg whites. Beat with chilled beaters until foamy and then add the remaining 1/4 cup lemon juice. Beat until stiff peaks form and then gently fold the yolk mixture into the milk mixture.
Pour the mixture into a baking pan and sprinkle with graham cracker crumbs.
Cover and freeze the cake until firm, roughly 4 hours.