Lame is a good description Have done as you suggest Andy. Used some Lead flashing which is easy to shape and adds packing, also carried out your bracket mod, no cracks were present. Why they didn't weld the belt tensioner bracket at the back of the box section I've no idea, plenty of thread and would give more adjustment.
That looks so much better! I agree with your comment about the tensioner bracket positioning. What an easy enhancement that would have been!