Andy, I used a 2x4 and it worked ok. I bought a 2x4 about a week before I started because it was real straight, but it warped. I was still able to use it though. It was stabilized by the time I put my forms on. Just make sure you have centerlines on the top and sides. Even though it was not straight I was able to line the forms up. The 2x4 strong back really gets its strength from the strips. Once you get a few srips on, I don't think it will move any more. A box beam would make lining the forms up easier though. A tip: The worst problem that I have had building my first woodstrip kayak is worry. I have found that little things don't matter that much.