Have a look at my first example;
You can't see much difference, can you?
Well, the one to the left is a 4:3 project and the one to the right is a 16:9 project. It's a bit hard to see the actual work area you have in your project.
Next image is a comparison of renders for the two different projects. First one is 4:3 with letterbox and the second one a straight 16:9 image.
Now I would propose a more defined work area where you clearly see where your borders are. In the following mockup I have defined the work area with pink colour and the area outside it with a grey checkerboard pattern. Not very pleasing to the eye but you get my idea.
I'm open for other suggestions, doesn't have to be pink on ckeckerboard, could be checkerboard on pink, I'm flexible.
p.s. The little racoon is from a CC-licenced short called "Doljer" by Adam Procter.