[Tom Line]

You absolutely cannot mount your bumpers inside the C-Channel. The bumpers must be mounted to the outside perimeter of your frame, with no indentations or recessing.

You will need to read R07, specifically section K, and understand exactly what the term "Frame Perimeter" means, it is definied on page 9.

By definition, your recessed areas are not exterior vertices. The bumpers may not attach to them or lay against them.