When i got the box it wasnt a perfect fit ( mostly as tolerance is tight in this spot with such thick ass material). so i used a 3" angle grinder and took off 1/16 from the bottom weld of the shock mount and the opposing outer top edge of box. just enough to get it in but still snug .
wish i could post pic but im sure youll figure it out. just spot weld on corners and run one bead along where shock mount arm contacts box , paint and done . was sweating it at first but ive spent more time polishing a single riser .