When I rebuilt my supports, I copied exactly how the original one was made. There is some pieces in the middle to make it rigid, but that might not be clear in the photos. I did end up installing these supports a few months ago, and it really strengthened up that side of the trailer and created a better alignment for the top and bottom shells. Since the trailer originally had a closet to the left of the door, we made a second support (the one with the cutout for the wheel well). This second support I think is important because the closet was not offering a lot of strength.
Depending on the model, metal supports where used instead of the wood one.