Fun! Prototyping this kind of thing and figuring out how to make it work is great.
Try a flexible cutting board for the stiffener, or maybe some foam stiffener (?)
Here is my seat bag write up. I just made seat bag V2 this weekend since this one sticks up in the back a little high. V2 just changed the angle at the seatpost a bit, trimmed a little off the top of the cutting board further back, and came out better aesthetically. I haven't tested V2 yet but it should be fine. I haven't decided yet if or how I'm going to connect the cutting board to the bag.
http://www.bikepacking.net/forum/index.php/topic,624.0.html