JK---When I get access to the trailer next spring, I'll post some pictures. For now, a thousand words or less. The box is not attached to the skin of the trailer. The flange on all four sides of the bottom of the extension is attached all the way around the perimeter. The opening at the top is narrower in the fore and aft direction than at the bottom. That is because the rearward most wall is vertical, while the forward wall rises parallel to the skin on the rear of the trailer, thus making the opening smaller. In structural terms, the extension is a cantilevered beam with its anchorage at the attach to the bumper. The air gap between the box wall and the trailer skin is approximately 1 and a half inches. Just enough so that when the lid is opened, it doesn't come in contact with the skin. Hope that helps.
2003 22ft CCD Int'l
2005 GMC Yukon Denali