Thanks

I always go into a deep research dive before I begin any of my builds, and I found these figures
Both feature a flat surface on the rear. The one on the left is a 1/6 scale Sideshow figure by ASMUS, and the one on the right is from ToyBiz and was released shortly after the film.
But I understand that these are figures/toys were created by non-film companies, and that is why I aways let the films be my main source of reference.
When Boromir arrives at Rivendell he appears on horseback with the shield strapped to his back. The woodgrain appears to be flat and does not feature any curve.
The scene where the fellowship is coming up over the hill when the ring goes south and we have the fullness of Howard Shore's score come through, we see the back of the shield again and the woodgrain is still straight.
Then when Frodo lost the ring in the Redhorn Pass and Boromir returns it to him, light reflects on the shield and we can see the back is flat, because the reflection is even and doesn't have a gradient as it would on a curved surface.
And in this behind-the-scenes photo of Sean Bean on horseback, we get another look at the rear of the shield with reflection in different light and the reflect shows a flat back again.