Pour evaporated milk into a small metal bowl, cover and refrigerate at least 1 hour.
Meanwhile, in a large bowl, dissolve gelatin and stevia in boiling water. Stir in lemon juice. Cover and refrigerate about 1 hour, or until syrupy.
In a small bowl, combine crumbs and margarine; set aside 2 tablespoons for garnish. Press remaining crumbs into bottom of a 9- x 13-inch baking dish.
Beat evaporated milk until soft peaks form. Beat gelatin until tiny bubbles form. Fold evaporated milk into gelatin, then pour over crust and sprinkle with reserved crumbs.
Cover and refrigerate 6 hours, or until set. Cut into squares.