Thoroughly mix crust ingredients and pack into a ten inch pie pan. Hint: try to get the crust thickness very even and not too thick. Puree watermelon to measure 2 cups. Add powdered sugar. Set aside. Soften unflavored gelatin in cold water. Add boiling water; stir to dissolve. Add lemon juice and pureed watermelon. Refrigerate until mixture begins to thicken. Fold in egg whites and whipped topping. Pour into pie shell and refrigerate until firm. Garnish with whipped topping. Makes one 25 cm or two 20 cm pies.