Combine sugar and cornstarch in a saucepan over medium heat. Stir in cold water and cook, stirring constantly, until thick and bubbly. Stir in cherry pie filling and almond extract; set aside to cool. Preheat the oven to 350 degrees F (175 degrees C). Mix flour, oats, brown sugar, and salt in a medium bowl. Cut in margarine with 2 knives or pastry blender until mixture resembles coarse crumbs. Press 2 cups crust mixture into a 9x13-inch baking dish. Scatter rhubarb over the crust; spread the cherry mixture over the rhubarb. Stir chopped nuts into the remaining crust mixture; sprinkle on top of the cherry-rhubarb mixture. Bake in the preheated oven until the top is crisp and golden, about 40 minutes. Yields one crisp.