You need to get the pvc warm in order to flatten it out. Cut down one side of your pipe section and then heat it up until it lays flat. Afterwards, sandwich the hot pvc between two flat surfaces until it cools off. It should turn out nice and flat. In this video you will notice that I used a heating element out of an old electric rotisserie along with a heat gun. An oven would work great too but I don't think I would use the same oven that I use for food.