I flattened plastics 2 different ways:
1. Put them on a wax paper lined cookie sheet in the oven at about 170-180° until they are pliable (5 minutes or so) and then put between 2 heavy books until they cool.
2. Use a heat gun on low (be very careful) and then flatten between books or a couple pieces of wood.
The plastic you showed should flatten out on its own to some degree when you heat it up....